It's about the Tarahumara tribe in Mexico who run in sandals for sometimes days at a time - for fun. It's a great read and this is my favourite quote so far:
"You can't muscle through a five-hour run; you have to relax into it, like easing your body into a hot bath, until it no longer resists the shock and begins to enjoy it."
We've all been there before, haven't we?!