Why are Kenyans the best distance runners?

Kenyan long-distance runners are arguably the best in the world. Is it the diet, the lifestyle - or anything to do with running barefoot to school? Elite marathon runner and Kenyan athlete manager Tom Payn and up-and-coming Kenyan athlete Boniface Kiprop Kongin tell Adharanand Finn, author of Running with the Kenyans, why their country has produced so many great runners