Free writing is great. The more you do it, the better you will become, and the longer you will be able to write. It's amazing how much you can write in a 20, 30, 45 minute period without realizing the time that has past.
Another suggestion I'd strongly suggest you look into is meditiation. Even simple breathing excercises are a simple form of meditation, and are not also good for the mind, but also the body. Also a great way of taking 'power naps' and being able to 'wake up' whenever you want.
Cheers,
Matt