Here’s a great post about what NaNoWriMo is great for, and what it is not. It will help you create a first draft and get down unexpected ideas. A draft like that will need many revisions. Leave your finished work alone for a while at the end of the month. Then go back and start the work of reshaping, adding, and cutting that will make your story work.
NaNoWriMo Is for First Drafts
One Way to Wonder 