In Bannister's day, no one considered 3:43 possible, and the 4:00 barrier was all anyone cared about. For this reason, Bannister trained not to be the best runner he could be, but just good enough to break 4:00. Still, it could be said that training for sub-4 actually made him better than he would have been without an ultimate goal. All that being said, what if someone just said, I'll do what Bannister did, except change it for 3:40. I know, eventually there is a physical limit for humans, but we've been wrong about that before. I mean, it would definitely take a much longer time to build up to, you gotta figure 10x440 at 55 would work just as much as with 60s.