"One step at a time and kilometers will disappear." -- words that helped finish 21kms of run in 2hrs and 57 minutes. So, how does this really work? No training, no regular work out and no preparation for the run. And still, you are able to push yourself to move your legs and hit the mark. It's bewildering but one thing someone can learn from this is that on any given day, your mental stamina beats physical fitness.
Your body can only be with you for 50% of the distance in case of long runs but what really gets you going is the determination and madness to just do it. Even though Nike said it but you have to JUST DO IT. The energy that one requires partially comes from the drinks and fruits but mostly comes from the spirit of the people around. The cheer and the moment has a lot more to contribute than your general health.
Of course if your body is weak you might collapse but miracles do happen and once you are a part of it, they seize to seem like miracles. Our heart+brain+soul has lot more than physical strength.
But again, without a precondition certain activities are impossible. However, beyond that "given" everything is up to our will-power.
Run for joy, run for peace, run for glory, run for nothing.