Quite amazing how so many people here cannot agree with the hypothesis that lighter, slimmer, longer legs will improve running economy and therefore assist with running faster times particularly over the marathon distance.
Let's take an extreme, oscar pistorius who wears carbon blades which are significantly lighter than a human leg. He can move those blades back and forth much faster enabling him to stay in contact with the ground for longer and also to use less energy - his heart has to work less for the same return of leg turnover. If you gave him human legs, he would be running significantly slower, the science of sport podcast said he would be not much faster than an average club runner. Certainly not close to qualifying for olympics. Thats how much of a difference it makes.
A kenyan leg and a white european leg are much more similar, but they are still noticeably different in most cases, and it is silly and illogical to say that the lighter leg will not have a competitive advantage in a long distance race.