Seeing all those magnificent marathon runners made me think about what drives them to take up such a huge physical and mental challenge. My dictionary defines motivation as "the desire or willingness to do something; enthusiasm" but that hardly begins to describe what it must take to run 26 miles and 385 yards around London on a hot day - or to train relentlessly week after week on lonely solo runs.
Motivation is about a whole lot more than desire, willingness or enthusiasm. It comes from deep within the most powerful recesses of the mind and is at the root of much of what we do - or don't do - in our lives.
The unconscious mind has a remarkable power to influence what we believe about ourselves and it's self-belief that is the driving force which empowers us to achieve amazing things- whether it's running a marathon, climbing Everest or losing weight.
Extensive research has shown that you can influence your own levels of motivation in a variety of ways and once you learn how to cultivate self-belief you quickly find yourself in a virtuous circle - the more you achieve, the more you believe you will achieve. Note that I said will achieve, not can achieve.
By attaching yourself to positive goals rather than "maybe I will, maybe I won't" targets, you're telling your unconscious mind to fire up the feelgood hormones that will drive you on to greater things.
That old saying that achievement is its own reward is so true, because our minds are programmed always to head towards pleasure and the great buzz we get from achieving something is caused by the release of the hormone dopamine.
Many of the people I help with their weight problems get their dopamine rush from unhealthy foods and sugary drinks. My job is to show them what's possible and gradually restore their battered self-esteem so they start to believe in themselves again.
Role models are so important because seeing is believing. An amazing lady called Barbara Greenwood ran the London Marathon and is an inspiration for anyone who wrongly thinks they're stuck with the hand life dealt them.
Barbara, who is 50, had struggled with her weight since she was 12 and at one stage was a size 28 who found walking hard, let alone running 26 miles. Thirty years ago as a student she vowed to run a marathon but it was not to be.
Then she began to train her brain to think differently about herself and food. Using one of my Thinking Slimmer downloads she changed her attitude to food and exercise and within a year she'd lost six stone - and taken up running.
The real difference in her mindset is this: Once she hated her body because its size was controlling her life but today she respects it as an amazing machine that has opened up a whole new world for her.
So many people who desperately want to lose weight by dieting are doomed to failure because they don't believe in their hearts they can do it. After a few weeks of getting nowhere through the conscious effort of deprivation and starvation (or dieting as they call it) they make what to them is a logical decision that it's never going to work - and they fall off the wagon.
Here's the point: Motivation is not about the logical, thinking part of our minds. It's about emotion, passion, confidence, belief - all of which come from the unconscious side of what's in our head. That's the starting point for achievement.
Losing a lot of weight and keeping it off is not a sprint, it's a marathon. Just as you wouldn't run 26-miles without the right mental attitude, so you can't expect to drop two dress sizes or more without getting into the right mindset.
So challenge yourself and do things you're afraid of, for every little achievement will be another massive building block in your self-esteem. Be like Barbara, believe in yourself and make things happen.
Because life begins at the end of your comfort zone.