There is still a lot of natural talent there, like, the natural ability for your body to take that many mpw. Myself for instance, I weight trained for running and worked my ass off to improve. Hit 70ish mpw and stress fractured both femurs. Its a huge combination and a hundred and one factors that plays into that sport. The thing about running, compared to like Soccer for example. Soccer would be incredibly hard to go pro, especially because of its worldwide fame. If you get hurt or something though and miss a month, you can come back from that without losing too much. Running if you miss a month due to whatever reason, you could have just set yourself back a year or more. There are many hard sports to go pro in, I just think running would be one of the hardest.