I should probably start with a note of caution. If you're a first-time marathon runner who has yet to make it to the start line, please stop now and come back when you have a finishers' medal around your neck. Because, what I am about to say, you really don't need to know. Really!
Like all first-timers, I went into the marathon not really knowing what to expect. And, having come out the other side, I now recognise that not knowing is actually rather useful. I guess it's like talking about the actual birth with a pregnant woman. If they've already signed up for the pain, why make it worse?!
I couldn't be more proud of my London Marathon finisher medal and the fact that I might now (although not yet confirmed) be #oneinamillion. But, I also know just how hard I fought for that (admittedly rather lovely) piece of metal.
I should add now that running for charity or getting a ballot place is an incredible honour. So many people miss out every year and I do feel hugely grateful that I had my time.
But, here are just a few of the things I am so glad I didn't know before I put my trainers on that day...
1) It is a really, really long way: it is easy to get caught up in the excitement of the Expo and the hype surrounding the big day, but, while for many the London Marathon is a great day out, it's actually, for you, a very very very long run. High-fiving kids and running around with jelly babies is great... for about 15 minutes!
2) You are likely to run more than a marathon: this little gem, I discovered on Birdcage walk as I was heading to the finish. My watch clocked 26.2miles at 4hrs 56m, but the official time was 5.01.03 and 26.5miles. Should have paid more attention to the blue line on the road marking the actual distance.
3) It is the best and worst day of your life: I get why people call it the best day of your life. In achievement terms, it is pretty epic. When you have a hip full of metal and a superglued stomach, it is also pretty painful. 14 miles were amazing. 12 were hideous. The .2 was surreal. I remember spending 6 miles chanting 'no pain here', just to get through.
4) There are people with hands covered in Vaseline offering help around the course: at least I think it was Vaseline. I think they were officially part of the day. But, I confess, I didn't hang around to find out. I wasn't hallucinating.
5) The tracker app means your spectators know more about you than you!: I finished the course, expecting the big reveal, only to find everyone was already discussing my splits on social media.
6) The gel stops make you feel like you are running through treacle: they sound like a real treat, but the gels make the road sooooo sticky that lifting your feet becomes a real challenge.
7) People might offer you drink on the way round (yep, a real alcoholic drink): a bit like torture at mile 23! I should also add that I was covered in holy water at one point and felt like I had been invited to a good number of house parties. You certainly don't need music to run with.
8) There are so many stretches without crowds: there are times when I felt that maybe I'd gone in the wrong direction.
9) You do actually care about that time: I told myself it didn't matter, but I did wear two pace bands (both of which I exceeded) and it did make me sad when I fell behind. 5.01.03 means I certainly have unfinished business.
10) You may get overtaken by a man carrying a washing machine: oh, and a rhino, a dinosaur, a canoe and an 85-year-old man.
11) There are now way too many pictures of your grimacing face in circulation: my running face is not something anyone should ever have to experience.
You might think that the combination of pain, embarrassment and general shock would be enough to make me hang up my running trainers for good.
But, with the amazing support, the knowledge that my cancer-scarred body with its dodgy legs can go the distance and the fact that I got to run through the beautiful city of London in a Breast Cancer Care vest made it all worthwhile.
Was it harder than I could have possibly imagined? Absolutely. Would I do it again? Absolutely.
Watch this space...