More Nanowrimo

I still have no idea what I'm doing for Nanowrimo yet. I still have 12 days or so before it starts. I've written stuff in science fiction, fantasy, horror, action. I have no idea what do write in. If I should just wait until November 1st and start writing the first thing that comes to my head. Or I could just pick one at random. I've thought about just working on one that I already have started. I know you're supposed to start a new story I think. But if Nanowrimo gets me writing on the book I started 4 years ago then I'm going to jump on it. I already have a bunch of scenes worked out. So it'd be easy to get going on the rest of the story. Maybe I'll be able to hit 50,000 words this year!