Deciding to do something as a "primary hobby' seems like a strange way to start out, but then maybe I should give that a try if it works for you. Generally speaking people do something because they enjoy it and because it meets certain specific goals. So my suggestion is to come up with goals for your maps and then draw to fill them. For example; you decide to make an RPG setting so need to map areas pertaining to your setting. The other approach is to say I'm going to make something and then, perhaps, find a use for it. Like saying I'm going to make an island now and then just getting down to it. Really both of these things can be used in conjunction, for instance if you are stuck or can't think of a project, you just do some random thing such as in the island example, and when you want to do something important to you you have the practice under your belt.

There is also this fact: Quantity leads to quality. So just keep bashing away at it.