President Harry S. Truman said:
"In reading the lives of great men, I found that
the first victory they won was over themselves...
self-discipline with all of them came first."
The same can said of any successful athlete or coach. I spend a lot of time reading and studying successful coaches. One common theme they have is self-discipline. It may not correlate with how they act away from the job, but when it comes to the job they are extremely well focused.
The same is true for any good athlete. There has to be a high standard of discipline and understanding that they will pay the price and be dedicated to their particular sport. It takes a tremendous amount of self-discipline to get to the top.