More often than not we aim for freedom from constraint. No one likes to be nailed down to a particular way of doing things. But on some occasions, constraints can be freeing.
For one, deadlines are great motivators. We'd never get anything done without any pressure. Writing is difficult enough — put a cap on it and all of sudden the words begin to flow.
Nothing resonates more loudly than an assignment, given by others or at the order of ourselves. Both require discipline.
So if you ever stuck on anything, establish an arbitrary count and then get in there and do some stuff — whether it's working out, shooting hoops, or writing a book.
Discipline is everything.