Several reasons why the Africans have an advantage over Western runners (even before they begin serious training):
1. They just plain live harder lives. Most of the elites come from the most rural areas, and have tough hard childhoods that starts them mentally and physically to accept and understand physical discomfort. VERY few US kids have that start in life.
2. Genetics. You have to be a complete fool not to see that its no coincidence that the greats come from the Rift Valley area and the Ethiopian highlands. For whatever reason ( and again quite possibly due to their rugged childhoods), they are able to withstand harder longer runs without breaking down. This in turn leads to a very diff. blood/muscle/capillary chemistry than anywhere else and they are able to handle higher workloads with minimal breakdown.
The average African teen has far more walking miles (if not running) in his legs than the average US teen. They may be junk miles in a sense but they play a big part in adapting the body for later stresses.
3. A Sink of Swim attitude. Almost every US runner has a way out, if running doesn't work out for them. For the Africans its just not an option. You can talk all you want about mental toughness in training, but economic realities are a very real part of that toughness.
4. Adult lifestyles. Even as professional runners they are very willing to sacrifice most creature comforts, and practice their craft. This has to do again with both economic realities and culture. There really isnt a lot to do in the villages except sip tea or work the land.
5. Fearlessness: Both in racing and in training. They are convinced that they are the best, and history backs them up. With this certainty in mind they go hard in training and racing.
6. Drugs? Forget about it. The vast majority of them wouldnt even consider it for a second. Why should they? The average clean Kenyan could beat the average doped American.