Rachel,
I have run a marathon before - once - that's it! LOL. But it was one of the most reqarding experiences of my life. Mentally, physically, spiritually - it transformed me into a new person. Hard to believe all of that can come from running a race, but its so much more - the training, the discipline, the friendships you make with running partners, it can change your life!
With that said, I don't know where you live, but you may want to look into USAFit. They are an organization that sponsors marathon training programs across the country. Here in Houston, they sponsor HoustonFit along with others. Check them out.
www.usafit.com. I think they have online programs too.
Runnersworld also has great advice for for begginer marathoners as well.
And also, a great "lessons learned" for me, if I had to do it over again, and I'm sure I will as soon as I'm done with school, is to focus on two things: stretching/yoga, and upper body development. Why? One, stretching is so important to preventing injuries. I can't tell you how many folks, myself included, almost gave up running because we thought we had bad knees or bad ankles and couldn't run long distances. Come to find out all we needed to do was learn to stretch properly! Yoga once or twice a week will help keep everything limber. Second, you will never realize how ever your head really is until you have run 18 miles of a marathon. Help this by building your chest, back and shoulders. Your legs will get plenty of work from the running, but teh upper body often gets neglected. You don't have to get buff, just strong!
I wish you the best of luck and have fun!
Joy