After reading this thread, I'm struck by how little we know the exact causal factors and the relationship amongst them that provide for exceptional long distance running. To take a few simple questions: How much does low body weight or body shape matter (and correspondingly how much is this a genetic matter)? Does it matter more or less and how much more or less than one's capacity to efficiently use oxygen (and how much is this genetic matter)? What about fast/slow twitch muscle fibers?