If you ask Kenyan runners why they are so fast, you usually get what feels like an unsatisfactory answer. It's all down to hard work, they'll say, or they'll talk about having a passion for running. But surely there's more to it than that.
At the 2011 World Track and Field Championships, Kenya won an incredible 17 medals in the middle and long distance running events. The U.S. won three.
In marathon running, Kenyans are even more dominant. In 2011, the world's top 20 fastest runners - in the world's most universal and accessible sport - were all from Kenya. Something is going on up there in the Rift Valley, where virtually all of Kenya's runners originate. So what is it?
A few years ago, the world 5,000m champion Mo Farah, from Britain, got a glimpse of the secret when he moved into a house in south-west London with a group of top Kenyan runners. The experience of living with the athletes changed everything for him. At the time Farah was struggling to even make finals on the world stage. A few years later he was world champion. "To see them just eat, sleep and train and nothing else was a big shock for me," he said in a recent interview.
Farah was already the top British runner at the time, but even for him, the level of dedication he saw from the Kenyans was a revelation.
Yet up in Kenya's Rift Valley there are thousands of runners living like this, training with an intense, almost monastic focus. Every morning, in the town of Iten, where I lived and trained for six months, you can see them everywhere, striding past in a blur of faded wind jackets and Lycra, like commuters in any other city.
It is this intense dedication to running by so many that sets Kenya apart. But where does it come from?
Virtually every successful Kenyan runner is from a poor, rural family. From an early age they run everywhere. Daniel Komen, the world record holder at 3,000m, told me: "Every day I used to milk the cows, run to school, run home for lunch, back to school, home, tend the cows. This is the Kenyan way."
The life of a western athlete, we are constantly told, is one of hard work and sacrifice. But these things are relative, as Mo Farah found out. For the Kenyan runners, hard work is just part of daily life. In fact, in Kenya the life of an athlete is one of relative comfort. Eat, sleep and run. It beats digging the earth all day with a hand plough.
For Kenyans, their focus is sharpened by the success they see around them. Up in the Rift Valley, every village has its star runner, someone who has gone off to win a world title or some big city marathon, and returned with enough money to buy a plot of land, a cow and a big car. There are role models everywhere. The children look around them and say, when I grow up, I want to be a runner.
So here you have a population who from a young age have been running everywhere, mostly in bare feet - which gives them perfect running form, and stronger feet and legs - and who all aspire to become athletes. Throw in the fact that they all grow up at altitude, which increases the blood's ability to carry oxygen (a good attribute for long-distance running), and eat a diet full of carbohydrates and very little fat, and you have the perfect recipe for producing great runners.
Underpinning all their efforts is the constant spectre of poverty. For every successful Kenyan athlete, there are ten others training in the hope of success. For them, making it as a runner, even modestly, is their only chance of escape.
Poverty exists in many other places, of course, and the will to escape it is not unique to Kenya. The difference, however, is that in Kenya, that will is channeled into running. Every last drop of it.
The importance of that will to succeed was clearly illustrated to me one day when I asked one of the Kenyan coaches why the athletes ran faster every time we came to a steep hill. My natural inclination, when going up a hill, has always been to slow down.
"That's because they see the hill as an opportunity," he said, smiling, knowing the answer would bamboozle me. "An opportunity to train harder, to run harder."
Many people point to Kenyan's dominance in running, and say it must be down to genetics. The big problem with this argument, however, is the lack of concrete evidence. Scientists have been conducting research on this for years and have so far come up with nothing. Unless they do, we'll have to concur with the words of Brother Colm, a retired Irish priest living in Iten and one of Kenya's top running coaches.
He says: "You want to know what the secret is? That there is no secret."
No secret, unless you count an incredible level of dedication, borne out of a hard, physical life that, as Brother Colm puts it, "makes them strong, disciplined and highly motivated to succeed." So motivated that every hill represents an opportunity. No wonder we can't keep up.
Running with the Kenyans was published by Random House on May 15.
The leaders in the Eldoret half-marathon in Kenya.
Athletes train on the track in Iten, Kenya.
Barefoot schoolchildren line up after finishing a cross country race at Salaba Academy in Iten.
Runners set off at dawn in Iten.
Athletes compete in the 3000m steeplechase event at a track meet in Iten.
A runner competes in the Lewa marathon in Kenya.
Author Adharanand Finn on a training run with runners in Iten.