You'd be amazed at what editing will add to your word count - and the goal is 50,00 so maybe it is National Novella Writing Month! Don't freak out over it, is what I say - worry about getting it written, first, and then when you edit you can flesh some things out, or add something if you feel it's lacking. You have a month to get it written and finished - well, by the official guidelines, I think - and editing can do wonders. That's what December is for. :)