I just finished my first marathon a few weeks ago right at 3:30 and I'm 45. With your background you could do it and probably be near 3:20 (or faster).
Most of the advice has been about putting in the miles and I'll partially disagree. One long run a week is plenty. You have some time but work up to at least a couple of 20 milers before the marathon. Equally important is training much faster than you currently run. Threshold and tempo runs are the way to go. Make your goal a 20 miler at a minimum of 7:40 pace and three two mile repeats at 13:40 with a half mile jog between them. You can do all this on less than 40 miles per week.
For motivation I'd check into running clubs or even join Team in Training. Some clubs are better than others but in some there is quality coaching and better runners to train with.