doesn't discipline require willpower though. I was told relying on willpower is bad because you only have limited amount of it per day and surely going to fail your when you run out of it.
Regardless of whether one accepts the arguments in the comic, I doubt willpower can be trained. It's not like a marathon where you have to actually run to practice, every day you're faced with decisions and temptations that you can "exercise" your will power with. In a sense you can't really stop practicing with your willpower...
This keeps getting repeated but there is no physical evidence to it. We know muscles have a limited amount of power (because they run out of ATP energy) but there is nothing that suggests it's the same for willpower.