Hard work can also just be mentally exhausting, and it's much easier to just wait for a proper creative mood than to struggle for who-knows-how-long trying to do something you know you can easily do with far less effort at a later time.
It's about great people and how they structured their days. What seems to be fairly consistent was that they were often 'on and off' as well, but to try to ensure that they did actually get work done consistently was to work a) every day and b) not for too long. So each day wasn't a slog like the usual 9 to 5 treadmill, but each day gave them enough space to get some of their best work done and have enough rest to be able to do it every day of the week, every day of the year.
Raymond Chandler had an interesting take on this. Basically he set aside four or five hours each day, during which he did not have to write, but he was not allowed to do anything else constructive. He could lay around or stare out the window, but no reading or paying bills - it was write or do nothing. Everything else, he said, came of itself.
