Even if you could isolate all the genes of top runners to a certain geographical region, it does not complete the hypothesis that genetics is the reason.
50% of all the professional hockey players comes from a very small population - the country of Canada. Does this assume that Canadians are genetically more genetically able to play hockey or are there perhaps other cultural or geographic reasons for this to exist?
The numbers of top athletes in the region participating in the sport geometrically increases the likelihood that the ultra talented athlete (Bekele) will be found.
By the way, to suggest that Kenya (or a certain region of Kenya) produces most of the fastest runners in history, is ridiculous when you look at the all time lists in the 10,000 and 5000m.
Bekele and Gebrehselasse are from Ethiopia. And by the way, the fastest this year are a genetic East African from Somalia (Farah) and a European (Rupp).