I think GTD is a bit wishy-washy about this. Putting it in your "hard landscape" seems to me to go against the principles of GTD, but I think David Allen recognizes that in his books as a good technique. Leaving it to compete with other actions on a context list may allow it to be over-ridden by a series of short-term urgent things.
Here's what I do for physical exercise. I decide on a certain amount per week that I plan to do, and I allow myself to do it whenever I can find the time. When I finish all the exercises for the week I give myself a reward. I also have one "overlap" day when I can either finish the exercises as the last day of the week, or get ahead on the exercises for the coming week. Somehow the existence of that "overlap" day, with its possibility of getting ahead, helps a lot to motivate me all week. This system has worked for me in the past and is working now, but doesn't seem to recover well from falling off the wagon. In theory I set myself reduced amounts of exercise when sick; in practice I guess I've been forgetting all about exercise and then having to remember to climb back onto the wagon somehow.
For music: I think there are studies showing that people tend to do well if they practice regularly in the morning. I used to wake up to the sound of my late sister practicing on her violin. I'm not sure if practicing in the morning is necessarily better or if it's just that the type of person who would want to do that is also likely to do well in music, but it's probably more useful to assume that it does help.
One technique I like to use is to learn the last bit of a piece of music first, and gradually learn longer and longer parts always ending at the end. This has advantages -- you always feel as if you're getting to more familiar territory, rather than as if you're always struggling uphill, as you would feel if you had learned the first part of the piece first.