Steve, great point about the programs. They succeed because they get enough bodies to throw into the meat grinder that they will have enough come out to form a team, whether football, running, or any other sport.
I recall a D3 school where they would regularly have 100+ guys at the beginning of the season. By the end, there might be 20 left but those 20 were in killer shape. Either you survived the workouts and came out in great shape or you broke down and either quit or were in the training room. It worked for them because they had enough guys to pass through the meat grinder.
Wilson, I do think there is a mental toughness aspect but I think it's what Coach Gambetta calls mental discipline. It's that ability to push yourself, at the right times, beyond where others are willing to go. The key is to have that athletic intelligence to know when the right times are.