I've never done NaNoWriMo. The concept seems a little weird to me frankly, but then I'm sort of a solitary beast. I don't work with a critique group or even a critique partner. The reason I'm mentioning it is that November is NaNoWriMo, so the activity starts in just a few days.
Some writers say they are always running into people who tell them they want to write a novel. I don't have that happen to me (maybe because I am a solitary beast?), but I think I've encountered two such folks in the last few weeks. I told them what I tell everyone--with apologies to Nike--"just do it." Just sit down and start the book and keep at it until it's done.
And then revise, revise, revise, of course, but we don't need to go there yet. The first step is to get something out there. Which I guess NaNoWriMo can help with, if the thing appeals to you.
The other myth that I like to dispel is that writing is fun. It may be for some folks, I suppose. I personally find writing hard work. Torture sometimes. Thinking about writing a book can be terrifying. I face doubt and despair each time I start a new project--which is where I am at the moment, except I do now have a first chapter, thank God.
Writing a novel is not a sprint, it's a marathon, long and grueling. And you'll probably hit many walls along the way. But, like running, it feels great to have done it--until you need to do it again, of course.