The thing to keep in mind about PROS, though, is it's not just a physiological thing. Even if you move up, meet directors want you in the event you're known for, especially if you're a championship medalist. Would you take a pay cut to run a longer distance? Another thing to consider is, while many athletes talk about moving up, very few are successful at it, especially middle distance runners. Those that do almost never have equal success at the longer distance. Lagat is one of the few that has had success, but he's pretty clearly not a 10k runner. There's adaptation but you can't your basic physiology. If you could through training then Usain Bolt could be a marathoner. Also, moving up takes time. People tend to stay in an event as long as they're successful. When they stop having success, they start talking about moving up, but by then they are near the end of their competitive prime in ANY event. Lagat could train just for 10k for several years, but aging would take away more than specific training would add.