I ran the Austin Marathon this year. I ran this one because it was my husband's first, and we wanted to run together.
I used the New York Road Runner's training program, available online. Mostly consisting of one long run a week, up to 20 miles, and shorter runs in between.
The week of: rest, carbs, and lots of water.
It was worth it to run all through my city, and see all the people who came out to support. The only downside is that my immune system was weakened by the run, and I got sick for the week after. Despite that, I will continue to run marathons.