Discipline is hard to spot. Sometimes a habit that makes you sweat looks like discipline. But it might not be.
Imagine an example: A young girl grew up scared of getting fat. Around 15, she started running daily and restricting her eating. Now 35, she runs six miles every morning and avoids sweets. From some perspectives, she looks very disciplined.
Zoom into her life. Whenever she has an engagement that's too early in the morning for her to run, it throws her off her guard. Without the run, she gets cranky; and she tends to avoid big social dinners. At some point, when her habits start restricting her social life, her therapist recommends she change her routine. Maybe she should take a day off each week. Or, he suggests, she might consider running less.
The therapist is asking her to leave her comfort zone, to exercise self-control or discipline. We typically define discipline as the ability to give up immediate pleasures for long-term goals. Our runner's situation is just atypical because her immediate pleasures are not the most common pleasures, like cake or beer.
But anyone who's struggled to give up a habit (whether it's ingesting too much or too little) knows that change requires an uncomfortable identity shift. In this sense, perhaps true discipline (rather than habits of rigidity) is your ability to leave your comfort zone. (My college roommate once said, "You write every morning--but that's not discipline, because you like it!") Indeed, discipline is related to the flexibility it takes to abandon a immediately-comforting behavior for a more frightening one.