Have you ever heard of the Pronghorn Antelope?
They aren't really antelope; their sight is about eight times more acute than ours, even as they have a 320 degree field of vision. Also, they're able to run 10K in about 10 minutes.
Scientists really have no idea why; the only faster land animal is the cheetah, who are native to another continent and are sprinters anyway. No North American land predator even comes close to the speed, let alone the staying power. In physiological terms, their oversized lungs and heart give them a Max VO2 of about 350; roughly seven times that of a decent casual athlete.
Why would they develop to be some of the best runners in the animal kingdom? Usually there's some easily explainable rational need. Usually some set of circumstances, once explained, make it all understandable.
I'd like to speculate that it developed these marvelous qualities because somewhere up the evolutionary tree someone decided that it's fun to run. It isn't always about running away from predators. It isn't always about getting the trophy. Sometimes, it's just about being out there pushing yourself because of the sheer joy of movement.
There are some great races coming up this summer; Flirt with Dirt, Dances with Dirt, the Crim, Labor Day 30K, Towpath and the Freep. I'm looking foward to all of them. But I very much rejoice each day as I'm able to head out the door, ready to conquer another run in the life, one breath at a time.