I, too, am a "first draft in my head" type writer.

Usually what happens is that I tell myself the story when I'm falling asleep or driving somewhere. Over and over and over again. But usually not the whole story; usually just the most interesting (to me) scenes of it. When I write, it's a matter of stringing those scenes together ... except that by the time I actually get there, I things may have changed enough that the scene I've been working on and polishing for who-knows-how-long doesn't go in the fic at all.