It's a long way down. We're parked at the edge of the Rift Valley, looking out over the vast, faded mass of land stretched out below. In the back of the van are about 10 Kenyan runners, including David Barmasai, who a few months ago won the Dubai marathon (the most handsomely remunerated race in the world) and is part of the Kenyan team for the upcoming world championships.
The driver starts the engine and we bump along the uneven dirt road winding its way down into the valley. Far below, small, cone-shaped mountains poke up like grassy knolls.
Forty-five minutes later, we come to a stop at the bottom. From here, the cone-shaped mountains tower above us, tiny farms perched somehow on the steep edges. We gather together for a group photograph and then, without any ceremony, start our run back up the dirt road to the top.
The Fluorspar run is a rite of passage for any runner in Iten. Most of the athletes here have done it at least once and like to compare best times. The record is apparently 1 hour 23 minutes. It's exactly 21km – a half marathon – from bottom to top. Uphill the whole way: a rise of around 4,000ft.
If the challenge of running up a 21km hill isn't enough, my day began at 4.30am when one of the other runners called me to ask what time we were leaving. He was ready to go, he told me. Soon after, we were all packed into the van and heading off through the darkness. The other athletes chattered excitedly as we went, while I closed my eyes and tried to get some kip.
When we start the run at the Fluorspar mineral mine, the morning sun is up and it's already fairly hot. After about five minutes, I'm drenched in sweat and starting to lose touch with the other runners. Hill climbing has never been my strong point so this is going to be tough.
"Sure and steady," I tell myself as I pitter-patter along, avoiding the biggest stones and trying to take the shortest line around the innumerable switchbacks. But whenever I look up, the others are further ahead, until soon they disappear completely.
People stop to watch me as I pass. At first they're friendly and I greet them happily. I'm feeling fine, just taking my time. But as we go on, I start to feel faint. I don't know if I'm imagining things, but everyone I pass seems to be laughing at me. I try laughing back, but it just makes them cackle even more. It's like a bad dream: the manic laughter, the dirt road that never ends, the aching in my legs, the pounding sun.
At one point, a lorry slips by – except it's moving barely faster than I am, and so for about five minutes to be following me, its straining engine growling at me to move aside.
Up and up, back and forth goes the road, rising above the cone-shaped hills into the cooler air, where the clouds cling to the chiselled rock face that holds back the highlands. Finally, with one last push, I reach the top. The rest of them are all sitting on the grass drinking lemonade and eating peanuts and boiled eggs. My legs are wobbly as I stand there, feeling like Edmund Hillary on the peak of Everest.
In the end it took me 1 hour 58 minutes, which isn't too bad. The others tell me kindly that anyone who can run it in under two hours is "very strong". They all, of course, completed it much quicker, in just over 1 hour 30 minutes. After all this time in Kenya, I still have no idea how they do it.
• The book Running with the Kenyans by Adharanand Finn will be published in 2012