This is an erroneous view. Ill go back to a specific point Hojd mentioned, about the genome diversity in Africa.
Marathon runners are from a very specific region of Africa, Kenya, Ethiopia and Uganda specifically, a region that has high altitude, and social accepted tradition of long distance running.
Now sprinters are either Jamaicans or Americans, this is because culturally that is because of the support locally that the sport has.
These two regions are very far apart, and have a much difference genetic make up. Jamaicans tend to be taller, same as Americans and that translate well to sprints.
Also if you look at the WORLD, the most popular sport (soccer) has a very wide race composition. From Argentinians, Brazilians, Africans and Europeans, The players that are good are all over the world.