Maugham is basically saying that it's discipline that matters. And once you do it enough number of times, it becomes so routine that you will automatically be able to write at the time you've set for yourself.
Yeah. What I want to add, from my own experience, is that you can have discipline (you show up) and still can't do anything (you are not prepared). This kind of dicipline in doing comes from discipline in preparing. At this age this could be even more important because we live in crowded cities that simply are not design for this.