Nows and Forevers

Three days from NaNoWriMo, and I’m still deciding on the story I’m going to write.
I’ve got three to choose from, where I’ve got enough in mind to see the beginning and a good chunk of the middle. I’m going to be surprised at the ending, no matter what.
I suppose I should be worried, right? I know plenty of people who have been outlining and preparing character biographies and what have you. But I have rarely been that kind of writer, and I’m not sure I’m going to start now.