One thing you might be mixing up is population vs individual. If you hold some sort of hiring process where the skill tested is similar to that tested in marathon running, then yes, Kenyans will tend to do better as a population. That does not mean you should actively discourage everyone else from applying. Your process should be blind to race/class of applicants, if some race/class does show a higher affinity towards specific skills, that will be the natural outcome of your process.

TL;DR - Don't hire only Kenyans, that's discrimination. You might end up with a lot of Kenyans if you hire only marathoners, but that's not the same.