Have you picked and registered for the marathon you are doing? If not, do that.
Also, go to watch a marathon...about two months before my first marathon I went to watch the finish line for about an hour at a marathon close to home. The excitement of the spectators and the success of the runners really is motivating.
Get good shoes...go to a real running store and have them assess your gate and running style. They will recommend the right pair of shoes based on your running technique.
Allow yourself a 16-18 week training program. Follow the training. I followed Hal Higdon's Novice 2 training program, missing only 1 run (4 days a week for 18 weeks...maybe it was 16, I don't know, can't remember) and I just about hit the 4 hour mark in my first marathon (4:02:02).
About a month and a half into training, your long runs should hit the 12/14 mile mark. I found a nearby half marathon to run as a substitute for the training run. I recommend doing this because it's real race scenario and you'll learn a lot about pacing. I did a half and it really put perspective on how fast (or how fast NOT to go) out of the gate. The last 2 miles of the half was incredibly painful and I cramped because I ran the first 4 miles at a ridiculous pace out of simple excitement and determination. When the marathon came, I approached that start a completely different way.
I'm sure there's more...but it's training, sleep, shoes, speed, commitment, discipline...and you'll be just fine.