We're constantly told that running will ruin our knees and outrage our hearts, but for nearly all of human existence, it was associated with freedom, vitality, and eternal youth.

In 'Tarahumara' land, there was no crime, war or theft. There was no corruption, obesity, drug addiction, greed, wife-beating, child abuse, heart disease, high blood pressure, or carbon emissions. They didn't get diabetes, or depressed, or even old: 50-year-olds outran teenagers.

If you can run six miles on a summer day, then you, my friend, are a lethal weapon in the animal kingdom.

But you can't muscle through a five-hour run that way; 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 created an unnatural form of running. It's not just the shoes, but we run on artificial surfaces - straight ahead, hard and steady - instead of speeding up and slowing down, reacting to the terrain with changes of pace and rhythm.

We've got a motto here-you're tougher than you think you are, and you can do more than you think you can.

If you don't have answers to your problems after a four-hour run, you ain't getting them.