Yes, it's a bit daunting. But there are things you can do to make it less so, and help guarantee your success (finishing the novel, not getting it published. That's a whole different story and numerous blogs.).
First, let me brag that I've never left a novel unfinished (whether it was good, again, another blog story) so that should give me some street cred to give advice. Additional bragging (hey, I'm entitled, I've worked hard) is that I have two published books, two more coming out, one in revisions with my agent, another book 2/3 finished and ideas for two others.
So, if you think you're just going to sit down and write that opus you're wrong. You'll get stuck at some point. And that's probably why if you're a veteran (lots of unfinished manuscripts in the drawer?), you know this is true. Before heading into that writing jungle, consider what I'm doing to avoid that:
1. Write a synopsis- about a page, no less. It will give you just enough detail to give you a general direction.
2. Do a character list. What do they look like, what are their names, their family histories, their jobs/grades, what's unusual about them, what are their talents, who do they like, etc. You don't have to know everything, and you may change things about them, but remember you're just getting started. The better you know your characters beforehand, the better you'll know how they'll react to whatever you throw at them.
3. Write out synopses for several key scenes or turning points that you know you want in the story; how your two main characters met, how someone was betrayed, a major fight scene. Sometimes it's easier to build around a scene (it's how some writers turn short stories into novels).
4. If you can, do a chapter outline. You aren't chained to it, but you won't get lost or stuck because you don't know where the story's going. Mull it over and if while writing your manuscript during NaNoWriMo you don't like it, or have discovered a different direction that is better, then deviate.
5. Do some research.I'm planning on writing a sci-fi story and need to find a suitable star system that has earth-like planets in the Milky Way. I've got to have information on why/how humans got into space. If you're going to write a western, you'll need to know what guns will be used, clothes worn, where the action takes place. By having information, you can refer back to help move the scenes along and add those very important details that give depth and fine tune your direction.
6. Build your world. My Sci-fi world will have humans, humanoids (look like humans but are not Earth-born), and non-human species. I need to know what they look like, do they trade in metallic money, does everyone have access to spaceships? What is the political system?Where does my story take place? As much detail as you can gather NOW will help you envision your story, and you won't have to stop and figure out the answers.
7. Make plans for when you can't write. Going to Great Aunt Henrietta's for Thanksgiving? Or having 25 people you have to shop/cook/clean for? You can write twice as much the day before or the day after, or on the following weekend. A lot of people fail because they didn't manage their time well and once they fall behind, they give up.
Remember, the purpose of NaNoWriMois to get you to write that book--from title page to The End. It doesn't have to be perfect (and it won't be), it just needs to be started and finished. Perfection will come with revisions (LOTS of revisions), along with more depth, character development and fleshing out.
So a little prep work now and it'll be easier (I swear!) to successfully complete your NaNoWriMo challenge. Good luck and let me know how you do, and if my list was helpful.
P.S. I'm working on my new Ultrabook with Windows 8 and still trying to figure somethings out- like how to shrink this page and access Word so I can put my fancy signature. Maybe next week...