From The New Yorker's conversation between Malcolm Gladwell and Nicholas Thompson, commenting on the World Track and Field Championships:
I watched the marathon and was struck (as I always am watching marathons) by the same dumb, obvious point: they are fast. It’s worth dwelling on this a moment. Back when Wilson Kipsang set the world record (which was then promptly broken), my running friends and I came up with the “Kipsang number,” which represented how long could you keep up with Wilson Kipsang while he was running twenty-six miles. I am a devoted runner and my Kipsang number is less than a mile. If I’m lucky, fourteen-hundred metres. You are a really good runner, and I’m guessing your Kipsang number is two miles. The average, healthy, athletic, American, twenty-two-year old varsity athlete in a sport other than track probably has a Kipsang number of between 400 and 800 metres. To recap: you could keep up with him for a quarter of a mile, then you would collapse in exhaustion. He would keep running at the same pace for another twenty-six miles.
My Kipsang Number? Around 200 meters or so.