Now we may think that some runners really are "natural" runners. Well actually we all are natural runners, see "Born to Run" and "Survival of the Fittest". The fact that some are much better than others is largely due to being given the opportunities to get these many hours of practice in at an early age. It is well known that Kenyan runners have dominated Marathon running for a number of years. This is not genetic, there is no difference in their genes than in Europeans. The simple fact is that they may in their early life run 20k a day at altitude and by their 16th Birthday would have amassed 6000 hours for purposeful practice. Killian Jornet climbed his first mountain when he was two years old. He wasn't "born" to do it he has just had much more practice than the rest of us.