Re: "why Blacks are such fast runners?"
Curiously, Kenyans - like the Japanese and some Mexican tribes with a traditional of long distance running - enjoy fermented food food stuffs. It isn't just their bodies, it is their fuel too that might impact upon their performance. Life is complex and fascinating.
In any any case, none of this means anything: anyone from any background is free to qualify for a marathon, and they'll only be judged by their performance. It's a meritocracy, with a clear and simple goal, judged objectively. In this respect a marathon differs to the reality of many jobs.