I think that the main difference is not genetic, but depends on the activity. A man needs 10-12 years for building his "aerobic house". If you are Gebre, your house can have 60 floors, if you are a normal athlete only 10, if you are an absolutely non talented person may be 3 floors. In any case, time that you have to spend for building your house is the same.
So, if you start running when are 2 years old, and more seriously when are 6 years old, you have built your "aerobic house" when are 18 years old. After that period, you can consolidate your aerobic level, but not to increase it very much. But you can work more in specific way, having a very big base.
Many times, my athletes rest 2-3 months for some injury or necause, winning some money, their motivation in running decreases. When they come back running, their shape is completely down, and they are worse than the most part of European athletes. But, having already their house, in very short time are able to come back to the same previous level, and from that moment they can improve using specific training.
So, the mistake that western people do is to consider only "official" training, without thinking that the base of Kenyans depends on all their life. In Nairobi, for example, nobody runs, and there are no athletes coming from that area.
The real problem is the difference of natural activity when you are young. If you think that 20 years ago, for example, two Portugueses were able running close 27 min, you understand that the problem is not regarding "current Kenyans", of sure less strong than 20 years ago but more motivated and better trained, but "current western people", that day by day is going far away from every type of physical activity.