As a relatively experienced marathoner, DON'T DO IT! The hallmark of every marathon training program is the Long Run, usually on weekends. The point of this is to gradually acclimate your body to long, sustained, multi-hour runs. If you haven't built up to at least 18-20 miles, your body is not ready. I understand that your goal is just to finish, which is the goal of most first-time marathoners, but it isn't worth the pain and suffering you will go through. Do you want to crawl to the finish line (perhaps literally), have a horrible experience, and swear of marathons or running for good? Or, do you want to train properly and have an enjoyable experience?
However, if you already paid the money for the race, it does seem like a shame not to at least run a little. So, here is what I would do. Since you mentioned that your longest run was 11 miles, why don't you start the race, and duck out at the halfway (13.1 mile) mark? That way, you get the experience of race day, you get a quality training run where water and snacks are taken care of, and you have a healthy, happy experience that gets you excited to run farther!

BTW, if you are looking for a great running website with training plans, I recommend Hal Higdon's site, www.halhigdon.com. He is a running guru here in the Chicago area, and he offers a lot of helpful information.