I'd say one of the more important things to keep in mind when writing a book is that books are pretty long. So that's probably at least several months to get even the rough draft written (depending on how fast you write, and how often you get stuck, and all that). And sooner or later, you're bound to get stuck and have no idea how to get moving without digging the plot into an even deeper hole. And you'll have to keep writing until you get it finished (because if you put it away too long, either you'll never get back to it, or have a terrible time finishing it if you do). This isn't to be mean or discouraging or anything, but just so you're prepared. If you don't feel ready for that yet, then maybe back off and write a few short stories first, and try to get those published. Or if you do feel ready, then just keep in mind that you need to stick at it, regardless of what happens. Even if you have to stop and start the whole thing over (which sadly, I had to do once, since I had chosen about the worst possible viewpoint character...). It helps a lot to make sure you have at least a basic idea of where you're going with the story. Know what the conflict is, know of a way to resolve it, where you want to start and end your story and how to get there, and know your characters as well as possible.
I'm not really sure how much of the plot you need to know, since I personally do terrible with outlines. I've tried mapping out stories in detail before, only to deviate in the second scene, leaving me stranded without a map. Since you haven't written much outside of RPs, though, I'd suggest making an outline in case you do need it. Then if it doesn't work, then at least you know how it's supposed to end, and if you do need it, then it's there.
Also, if all you want is just to get published, and don't need it to be a book, then I'd recommend at least considering a short story first, since there are lots of magazines out there looking for short stories to publish.
Sounds like a neat story idea, btw.
