I just completed my first marathon a couple of weeks ago. I started back running in early 2006 and couldn't run 1.5 miles continuously at the time. I decided to train for a marathon for motivation and did ok until shin splints shut me down in October 2006. I'd ignored the advice of several that you should run for a year or so before attempting a marathon.
I started back training in April 2007 and was able to run virtually injury free for the entire time I was training. I used the Hal Higdon Novice training schedule for my marathon and really liked it so I would second the suggestion from Zeke to check out his program. I am probably 15 lbs above an "ideal" marathon weight for me, so avoiding injury and not ramping up the mileage quite as fast as the quickie training programs do was a priority for me. Good luck with your training. GET GOOD SHOES FIRST.