Your "running genes" are so numerous, and so dependent on environmental factors for their expression, that developing into a world-class marathoner is like playing a hand of 50 cards from birth.
If you are a card player, you'll understand that with that many cards in play, nobody is dealt a "winning hand." Everyone has plenty of good running genes and extreme outliers are rare. It's how the genes are switched on and off during development, how the cards are played, that matters. It is still mostly random, because there's no way to control or even understand yet just how and why genes are active. But it is not a matter of having the right parents. Nobody has a decisive advantage at birth.
What western running fans fail to understand about the Kenyans is that there's 40 million of them. There are 4 million Kalenjin. Very few are distance runners and their favorite sport is, in fact, rugby football. There are a lot of big, fat Kenyans. But there are also a lot of skinny Kenyans, especially in impoverished places like the Rift Valley. Skinny people have a huge advantage at distance running. Poverty doesn't just give Kalenjin an incentive to go make a few thousand bucks on the roads, it also makes it easier for them since by their early 20's, they have grown up skinny their whole lives and are adapted to it. Contrast with westerners who are typically skinny only during a teenage or early adult phase, but fill out to a higher natural weight afterwards.
Watch Rupp's 10k AR and you'll see three naturally skinny east Africans pacing one American in a prolonged skinny-teenager phase. If Rupp had grown up a bit hungrier, he'd have looked like his pacers. Instead he looks like a guy with big bones that haven't grown muscles yet. His natural weight would be about 170.
Long distance running is dominated by skinny people. East Africa is home to a lot of skinny people and America is not. It's really that simple.
