I ran a marathon in October of 1998 -- the Marine Corps marathon. I highly recommend, first, that you find a race that will suit you. By that, I mean, if you're the sort that likes to run with other people, check past years' finish times and see if there are lots of people who finished in the same amount of time you think you'll be running. By that same token, if you're going to be slow, make sure the race you choose keeps the course open long enough for you to finish.
I trained by running 4-5 days/week. I did one interval workout (I can give you more details on how to do this if you want), one long run (start where you are and work up to 20 miles...I can give you more details about how to accomplish this as well, just ask
) and 2-3 shorter runs of 3-7 miles each.
I finished in just over 5 hours and it was one of the greatest things I ever did. It was comparable to giving birth, actually...something I definitely want to do again.
Feel free to ask me questions!