“There are people who put their dreams in a little box and say, Yes, I’ve got dreams, of course I’ve got dreams. Then they put the box away and bring it out once in awhile to look in it, and yep, they’re still there.” — Erna Bombeck
The whole point of NaNoWriMo is to stop saying, “One day” and just do the damn thing. Some folks say 50,000 words isn’t a novel, but it is; both Harlequin Romance and Harlequin Presents clock in at 50,000 words, while Harlequin American has 55,000-60,000 words, which is also doable in November if you get up a good head of steam.
The point is, your story doesn’t have to be a doorstop to be complete. But even if you know it’s going to go more than 50,000 words, just think how much closer you’ll be at the end of November.