I can only speak from personal experience, but I made enormous improvements on my own with a book in a pretty short period of time. I'm now at the point where I'm looking into joining a tutor-led group, because I think I've made most of the easy gains from pursuing it on my own.

If you're the sort of person who can, for example, hunker down and learn a programming language over a few hours under your own steam, I imagine you can probably do the same with drawing. For me, the experiences are quite similar.