Since I'm the one who spoke with Coach Larsen and began the discussion, let me pipe in here. Bob's intention in saying "anyone who can run a fast 10K can run a fast marathon," was not to say he/she WILL run a fast marathon, only that they would have the ability to run a fast one.
It's more a matter of taking the talent and training it for the longer distance. Not every great 10K runner has been able to manage that, obviously. Coach Larsen's own Meb K. has yet to fully nail a marathon from a time standpoint based on his 27:13 10K PR. But he still has the "ability" to do so.
Remember, it took Tergat six attemtps to win his first marathon. And Haile's coach, Dr. Woldemeskel Kostre told me in 2000 that Haile would be able to run a fast marathon, but first he had to re-train his 'sports car' body into a 'rally car' stride, because 10K racing consumes fuel too quickly to last through 42.2K.
That's why I suggested that Mo Farah's 3:28 1500m speed, if properly trained...