One answer is to like the grind. This may not be true for everyone but many world class athletes do feel that way. In the world of tennis, by the time they reach world class level where training begins around age 7 or so to age 25, they have been at it, almost on a daily basis for many years. Maintaining that effort is only possible if in some way it is rewarding. Recently I watched a dressage video for the world championship, (I like horses). The owner and rider said during an interview winning was mostly to to the horse which liked the training and moreover, liked performing and being the center of attention.
I watched a video of a ballerina recently that highlighted how she trains and what she eats. She eats plenty but also logs every calorie and trains daily. It is interesting that she stretches at every opportunity and rolls out sore muscles just as a cyclist would. The only way t o maintain such a high level of disipline is passion.