So many endurance runners spend their lives knocking off endless marathons - and though they probably think it's a stretch, it's nothing of the sort.
I guess it depends what you want out of life. Do you want to continue striving for the same goal over and over again? Or do you want to evolve?
That isn't to say that running isn't a glorious thing to do. Apart from perhaps sex, it is the quickest route to a legal high in 30 minutes' flat.
But if you're looking for ecstasy, and if you're looking to grow, then you're going to need a fresh challenge.
Ohhh, and one more thing. Not that this is of any import whatsoever, but... It might also pep up your conversation. Who wants to hear you blabbing on about another marathon? Who would even want to sponsor you for running another marathon?
What people want to hear about are new challenges. Challenges where you are having to start right at the bottom. And challenges where you might even fail. Then they'll be licking their lips to hear all about it. (As I know from my epic fail at the Hellespont last year...)
1. MUCK AND GLORY. Tough Guy, Staffordshire
There are two versions of this swamp-monster challenge, the original Tough Guy in January, when the mud is just a notch above freezing, or the summer assault which goes by the name "Nettle Warrior".
I tried the mid-winter version, and I can still distinctly remember the ice-cream headache of jumping from a five-metre board into this thrillingly cold pond.
There are other excitements.
Although I don't think that anyone has yet managed to kill themselves on Tough Guy, it is very easy to end up in hospital. One of the obstacles is like a giant clothes-horse, at least ten metres high. I was climbing up these huge horizontal telegraph poles, which were about four feet apart and greasy with mud. Directly beneath me was another competitor. He'd just broken his leg; his face was absolutely as white as paper.
What I love about Tough Guy is that, right from the get-go, all competitors have to sign this document which roughly says, "If I hurt myself, it's my own stupid fault."
The nutty organisers have never yet been sued and they seem to relish their annual accident figures. It is risky - and that's precisely why, year after year, people keep going back for another basinful.
At the start, there's a six-mile warm-up run, which includes the "Slalom" as you zig-zag up and down, and up and down, the same hill.
Then you get onto the mud and the glory of the Killing Fields. Favourite bit? So many to choose from! The Vietcong tunnels, as you crawl these drainage pipes in complete darkness; and clawing through a pit of mud underneath a barbed wire canopy; and, later, balancing along these narrow planks of wood which had to be a good 15 metres off the ground.
Most of our lives are now straitjacketed by the rules and regulations of Health-and-Safety and so we yearn for something like the daft adventure of Tough Guy which also offers us the distinct possibility of being able to do ourselves very great harm.
2. CLIMBING. Aonach Eagach, Glencoe, Scotland.
This is not a technically difficult climb. But it is a real test of nerve. After about half-an-hour on this hair-raising ridge, I found myself edging along a little track that was about 12 inches wide. On one side was a precipice of several hundred metres, and on the other side was another equally vertiginous drop. Even if you have quite a good head for heights, Aonach Eagach can fairly make your eyes water.
The ridge is about seven miles long, and on a fine day, the views out over Glencoe are spectacular. When I first started on Aonach Eagach, I was absolutely terrified. I remember gazing down into this bottomless chasm as I climbed one of the "Crazy Pinnacles" - and thinking to myself, "If you slip now, you're toast."
Halfway along the ridge, we came across a young 13-year-old lad who was out for the day by his dad. They were roped up, but the boy was petrified; worse, there was no way off. It must have put the boy off climbing for the rest of his days.
But once you get over the terror, it can be wonderfully life-enhancing. After an hour or so you may even discover that you're in flow - moving smoothly and calmly over the rock, and not thinking about the cliffs, but rather enjoying the whole experience of living in the moment, of knowing that your life is in your own hands. Doesn't happen so often these days.
Have a care when you're descending down to the Clachaig Inn. There are two routes, one direct down a scree slope, and the other much more circuitous. The scree slope can seem very tempting, but you need to know what you're doing. There are huge drops on either side, and it kills at least three people a year; fantastically dangerous when it's misty.
Even if it's sunny, you still have to take all your gear, as the Glencoe weather is changeable. You'll also need at least two litres of water. Book yourself in for the night at Clachaig Inn - it is homely and great value, not that you'll give a damn where you spend the night, because you'll still be fizzing from having walked the ridge.
3. SWIMMING. The Hellespont, Turkey
This epic waterway separates Europe from Asia and when you see it for the first time, and you realise that you're going to try and swim it, a little shudder zips up the back of your spine.
Lord George Byron - "Mad, bad and dangerous to know" - was the first of the real open-water swimmers, and he managed to swim the Hellespont on the second attempt. He said it was the best thing he'd ever done.
Not that I yet know what it's like to swim the Hellespont, as last year I managed to cock it up and ended up being hauled out of the water by one of the many fishing-boats, but anyway... I certainly know the theory of what you've got to do to swim the Hellespont, even if I haven't (yet) been able to put it into practice.
The swim itself is about four miles, but with the tide it's more like two. You're starting in Europe and heading off to Asia, and the finish-line looks absolutely miles away. You've got 90 minutes to finish the race. The main thing is that you've got to take the correct line. Let me explain.
The Hellespont is this vast 38-mile strait that connects the Sea of Marmara with the Aegean Sea, and on the surface there is a strong current flowing north-south. The finish-line of the Hellespont is at Canakkale, which also happens to be the very narrowest point of the whole strait, and it's at this bottleneck that the current is running full and hard.
And basically: when you're swimming out from Europe, you can't aim direct for Canakkale, otherwise you'll be swept straight out into the Aegean. Got to aim high, at least 45 degrees north of Canakkale.
As for the training itself, well if you're no good at swimming, then it's going to take about a year's worth of training and lessons before you're ready. Me? It took 18 months. And I still didn't manage it.
A lot of people don't like the experience of being a novice. They find it embarrassing to have to get into the pool and have their swimming stroke deconstructed down to its basic elements.
For months on end, I could only concentrate on one element of my stroke. I could either get my breathing right, or my stroke correct. It took ages before I could combine the two.
If you really are learning from scratch, you want to learn the Total Immersion method of freestyle, head totally immersed in the water. It's a long, smooth, gliding stroke that is perfect for endurance swimmers.
Of course there are any number of great outdoor swims that you could attempt. Those good people at Swimtrek will even lead you out for a jail-break swim from Alcatraz.
But the Hellespont has history. Behind you are the 1915 Gallipoli battlefields (centenary next year), and ahead of you is Asia and ancient Troy, and there you are in the middle of this raging strait, and the very thought of it is both so awesome and so ridiculous that it makes you almost want to laugh out loud.
4. DIVING. The swan dive from a ten-metre board
This was the dream: a holiday in some sunny spot in the Mediterranean with a handsome seaside cliff. A group of family and friends watches from the beach as all the local lads jump off the cliff-top and tombstone into the sea, but then an awed hush ripples through the expectant crowd as this middle-aged guy in floral beach-shorts climbs the ladder up to the top of the cliff. He's so high up that he is barely a pin-prick against the sky. He gives a brief wave - So dashing! So brave! So handsome! - and then with arms outstretched, he dives off the cliff, back arched, head high, practically a bird in flight, and then, his hands coming smoothly together before he executes the most perfect swan dive to enter the water with barely a murmur of a splash.
That, then, was my fantasy.
The reality was joining a whole load of kids at Edinburgh's Commonwealth Pool, and doing belly-flop after belly-flop until my skin tingled as if I had been stung by a thousand jelly-fish.
Even I was left just a little staggered at my own ineptitude. In fact, now that I think of it, this whole diving experience was more of an exercise in humiliation, as I yet again muffed another tuck-dive from the one-metre springboard while these eight-year-old kids were not only brilliant but utterly nerveless.
When you first look off the ten-metre board, it seems like one hell of a drop. It is one hell of a drop. And then you steel yourself, and say some life-affirming motto like, "Living the dream", and launch yourself into oblivion and hope to hell that all your training is going to pay off.
My speciality was the back-flop. Your legs go over too far and you land flat on your back, and off the ten-metre board... Oooh. Now that's gonna hurt.
But rather satisfying when you can manage to pull off a half-decent dive - though somehow the video playback never seemed to match up to my fantastical expectations.
5. SKIING. Engadin, Southern Switzerland; the world's greatest ski marathon
All the thrills of a genuine marathon - but without the gruelling wear and tear on your body.
Cross-country skiing is gloriously self-sufficient. No need to rely on lifts, or join a queue, or slum it in a sweaty gondola. Competent skiers need about three days to get the hang of it. The main kit differences are that the ski poles are longer, and in place of the robust ski-boot you wear dinky little shoes which clip in at the toe but which leave the ankles free to lift up.
The Engadin ski marathon has been going for well over four decades, and, like everything that comes out of Switzerland, is organised with absolute clockwork efficiency. The night before, there's a huge party, then free transport to the start and at the end there will be warm clothes waiting for you.
The start is Le Mans-style, which is nightmarish for the pros but good sport for the hackers, and then you're off, scything and slashing with your long poles, like an army of black ants nibbling away at the pure white landscape.
Engadin takes place in March, with over 10,000 people taking part in the marathon and half-marathon. The skiers are split into five staggered groups, with the day-trippers leaving last of all. The veterans take it easy for the first mile, saving up their reserves for the last few killer hills.
The last three miles, the Golan Heights, are a severe section of hills and are the very last thing you need after 23 miles of hard cross-country skiing. But you'll be cheered on by squads of eager Swiss spectators yelling 'Hopp, Hopp, Hopp!' It feels like you've enrolled in a beautiful army boot camp.
6. CYCLING. Rickshaw-rider, Dundas Street, Edinburgh
At first, when I was drawing up my top ten challenges, I thought that the cycling challenge should be something like L'Etape du Tour. It's got all the killer hills that you'd expect of a stage of the Tour de France.
And then I realised that there is, actually, something that's a bit more of a stretch than L'Etape du Tour: Dundas Street, Edinburgh.
With a fully-laden rickshaw.
As challenges go, this is middling-tough. I don't think I could do it. I'd love to see how our local lad, Sir Chris Hoy, would get on with this one.
The rickshaws are quite hefty pieces of kit in their own right - they weigh in at 100 kg. And then there's you, who might be around 70 or 80 kilos. And then there will be the three hefty blokes sitting in the back of the rickshaw, tippling away at their beer bottles as they watch you sweat and grunt your way up the hill. That's going to be well over a third of a tonne.
And Dundas Street is a pretty big hill. Even if you do have 27 gears.
Still - being a rickshaw-rider is a great challenge in its own right, though Edinburgh must be one of the hilliest cities in Britain.
The rickshaw-riders - the rickers - are a very welcoming bunch as they tinkle their bells at the weekend in the city centre.
And then you've got the whole business of haggling over the price and bantering with your clientele. That can also be a stretch.
Unlike most of these challenges, at the end of the weekend, you'll even end up with a profit. At 3am,when you collapse onto the couches with all the other rickers, and as you share you tales of derring-do, that first beer of the night will never have tasted so sweet.
7. ROLLERBLADING - Pari-Roller, Paris.
Pari Roller is a matchless midnight rollerblade through the very heart of the world's most romantic city.
But first, a cautionary tale for those saucy young pups like myself who are prone to tackling things without proper instruction.
This was the incident that cured me.
Manhattan's Central Park is the lungs of New York, combining acres of parkland with hilly woods, ponds and a handsome reservoir (Dustin Hoffman jogs round it in Marathon Man).
Along the perimeter of Central Park is a looping 6.2 mile road, which I have run round many times.
I have rollerbladed it just the once.
It is difficult to appreciate from the back of a yellow cab, but Central Park has a number of severe hills. The steepest of these, up in the north of the park, was known as The Mother.
Rollerblading was all the rage in the 1990s. I purchased a pair of blades and that very night gave them a spin in Central Park. I had no instruction, no training manual, no tips, no guide-lines; I had nothing but my pluck, my knee pads, and my hard-won fecklessness.
I set off from 72nd Street, grimly jinking from side to side, like an old man doubled up with arthritis. After a while, I picked up a bit of speed. I even stopped looking at my feet. It was like ice-skating only more painful.
And then, as so often happens with so many of my ventures, I got cocky - and my cockiness coincided with the arrival of The Mother.
I had not yet attempted to brake. Braking calls for a slightly technical operation where you tilt one boot up and effectively dig in the heel.
So my strategy for tackling the Mother was to 'schuss' straight down like a downhill skier, feet side by side, knees locked and bent.
It was going to be fast and it was going to be hairy. It was certainly going to get the old adrenalin pumping.
That was the plan. Simple yet effective.
About half-way down the Mother, with the trees whipping past in a blur of twilight green, I realised that if I crashed, it was going to end very badly.
The madness descended. There and then, on the steepest hill in the whole of New York City, I attempted my first ever braking manoeuvre on an inline skate.
Very delicately, I lifted up the toe of my right boot and started leaning back on the heel.
The scene must have been similar to Donald Campbell's spectacular crash as he attempted to break the world water-speed record on Coniston Water. One moment Campbell was serenely gliding across a glass millpond, the next, Blue Bird was cartwheeling 30-feet up in the air.
As for me, I managed one backward somersault before careening down the tarmac like a human bobsleigh.
I couldn't walk for a month.
And that is the reason why, if you're going to have a shot at Paris's Friday Night Skate, it is advisable to first have a few lessons in rollerblading. Braking, in particular - now that's a handy little trick to know.
But it's worth the practice.
Pari-Roller is the very best way to see Paris's sights - at night and at vast speed. It's free and it's held every Friday night, with up to 15,000 bladers congregating at Montparnasse before they hare off on a 19-mile circuit of the Paris streets.
The bladers meet up at 10pm and the police close off the streets until 1am. Other cities have tried to launch similar blading events, but Paris is the original wild one.
The organisers have only one rule and that is that you have to be able to control your speed. For a lot of the route, the bladers are as tightly packed as the Tour de France peloton and they're probably going just as fast. If you go down, you'll be taking 20 people with you.
But the Pari Roller is an incredibly cheap thrill. There are few other places on earth where you can skate for mile after mile on good tarmac - and through such majestic scenery. On a dry summer night, after a light dinner and a half-bottle of red... it doesn't get much better.
8. KAYAKING - Devizes to Westminster International Canoe and Kayak Marathon
They say that to complete the Devizes-Westminster, you need the speed of a race-horse, the stamina of a cart-horse... and all the brains of a rocking horse.
This historic 125-mile race has somehow turned the graceful, elegant canoe into nothing short of an exquisite torture rack. But the race is as gruelling as you want to make it - the roughie-toughies can choose to complete the whole course in a single non-stop race, while the saner souls opt for the four-stage tour which takes place over the whole of the Easter weekend.
I know which one my money's on.
The non-stop doubles is the Blue Riband event of the Devizes to Westminster (known to devotees as the DW). Just take a look at these people at the end of the race. They are shattered - completely spent, and will probably be spent for at least the next week. One competitor was so drained that even a fortnight later, he didn't have the strength to push a wheel-barrow and he complained, in a rather little-boy-lost fashion, that he was still waiting for the elation to kick in.
Added to this, there are all the usual ailments that come from a non-stop endurance event - in the case of the DW, bruised backs, aching fingers, tendonitis, chaffing and blisters the size of Loch Fyne oysters.
Opting for the four-day race will transform your whole experience. From being an excruciatingly painful slog, the DW becomes a bewitching Easter weekend.
The four stages comprise: 34 miles from Devizes Wharf to Newbury on the Kennet and Avon Canal; 34 miles to Marlow, during which you move onto the Thames; 36 miles to Teddington Lock; and then a 17-mile run up to Westminster, after a frisky mass-start on Easter Morning.
The DW is packed with dads re-connecting for the first time in years with their children; buddies having meaningful conversations with their mates; and spouses re-discovering the dynamic partners that they married all those aeons ago. The camp-site camaraderie amongst all the competitors is intense; on the Monday morning, the whole area feels like it's end of term.
9. SKATING. Elfstedentocht, Friesland, Netherlands
Skating as it was meant to be done: not ploughing in circles round an indoor rink, but out on the frozen canals and lakes of Friesland, northern Holland.
Most skaters never experience the fantastic liberation of getting away from the rink. We learn our twirls and our pirouettes, we learn to skate backwards and we especially learn how to skate round in circles. But we never get to skate al fresco when you can bomb along for mile after mile without seeing another soul.
It's a type of skating that takes some getting used to - it's like swimming on the high seas compared to pounding out lengths at your local pool
The ice can get slushy. The surface can be so cracked that a single lapse in concentration can send you tumbling. Low bridges need to be ducked. And sometimes you have to stop skating altogether and go through the indignity of walking on a wooden path.
Outdoor skating can be risky and uncomfortable, but it frees you from the dull, pedestrian cocoon of the ice-rink. It's living in the jungle rather than being safely cooped up in a zoo cage.
Elfstedentocht - The 11 Cities Tour - is a 120-mile loop of canals which starts and ends in Leeuwarden. As its title suggests, it incorporates 11 'cities', which are in fact modest towns which earned Medieval city-rights.
There is a time-limit - and a particularly mean-spirited one at that. In order to win the coveted Kruisje (a Maltese Cross with the Elfsteden insignia) you need to complete the course by midnight. This is fair enough - except that many of the locals are starting the race up to four hours before the foreigners. Talk about home-pitch advantage!
Thanks to global warming, there have only been 15 Elfstedentochten; the last one was in 1997. They're held at very short notice, and when they do actually happen, the whole of Holland goes absolutely crazy.
There is an 'alternative Elfstedentocht', for all those years when the main race is cancelled. The Stichting Winter Marathon is held at Weissensee, Austria, towards the end of January. Weissensee always has plenty of ice, and is also set over a 120-mile course - only this one comprises eight loops on a 15-mile circuit. It's not a bad second-best if you've put in months of training, but somehow it just doesn't have the romance or the history of doing Elfstedentocht's one enormous loop.
10. ULTRA-RUNNING. The Marathon des Sables, The Sahara, Morocco
This is held to be the most gruelling foot-race on earth - and yet for many it holds a quite spectacular allure. Of an evening, runners will sit at home and fantasize of Beau Geste, and sand-dunes the size of mountains and starry nights in the Sahara.
The race has a romance and a mystique far beyond any other endurance race. It is the whole Megillah, and just completing the Marathon of the Sands (MdS) is an extraordinary achievement.
Some of your friends may be impressed, though the truth is that the bulk of them will think that you are out of your tiny mind. Get used to being told that you're going through a "mid-life crisis".
The MdS is a week-long, 150-mile slog through the Sahara, carrying all your own food and kit. The French Commissaires provide you with leaky tents and water, but as for the rest, you're on your own.
Temperatures regularly hit 54 degrees. How hot is that? It's drinking 11 litres of water - and never once needing to stop for a pee. You're sweating out water as fast as it goes in.
After the first day, I remember that my sleeping bag was completely sodden. I thought a bottle of water had cracked open, and then I realised that it was my sweat, wicking off my back and into my rucksack.
The blisters on this run has to be seen to be believed. Runners' feet often swell by a couple of sizes. Serious infections are common-place - as are popped knees and wrenched ankles. Your toe-nails will be carried off like cherry blossom in a spring gale.
On the fourth day, the double-marathon stage, I saw a Frenchman have a heart attack. A helicopter whisked him off to hospital in Casablanca.
It is an exquisite fusion of pleasure and pain. But to any marathon-runner out there, it will always remain the ultimate challenge. And if you don't do it, then it will be a big miss. One day, forty years hence, you will be sitting by the fire, dreaming your dreams, and wondering why on earth you didn't give it a shot when you had the chance.