I find your NaNoWriMo experience interesting, because the one time I did it several years ago, it was the exact opposite: when I started I had not the vaguest clue where the story was going or even who all the characters were going to be. But by the time I reached chapter three the thing was practically writing itself.
Perhaps I was just lucky. This year I am going to plot the thing first, I think. But I find that knowing how the story is going to end tends to rob me of momentum: when I know what is going to happen it becomes a chore to write it all down.
I quite agree about not worrying about bad writing. One can always polish it later, but there has to be something to polish!