I believe that if a person comes from a background where their family members are athletically inclined, then they are more likely to be successful at sports. There are many examples right here in the US where brothers are both members of national sports leagues such as the NBA and the NFL. In some cases there are athletes that have had a parent who is former player in a national sports league. Genetics definitely do help although hard work and dedication is what truly make an athlete successful.
Although there is certainly a genetic aspect to the success of the Kenyans when it comes to long-distance running, the real key is their constant training. These guys train from the time they can run for these types of events. When you are THAT dedicated to something, you're going to have success at it.
One would have to truly study the subject to be sure but I'm very sure genetics alongside training are responsible. One can have the leg up of genetics but genetics can mean little to nothing without proper training. For example, men have more upper body strength than lower and for women the inverse is true but women are perfectly capable of besting a man at weightlifting if they train correctly and efficiently and vice versa for men when it comes lower body strength challenges.
No, genetics is not responsible. The terrain of Kenya helps shape a runner's ability far more than genetics. Good runners must train every day and those who are given tasks like running from one town to the next will become some of the best runners over time. It is almost always about the training.