Train well in advance. I ran one after a cross country season and three weeks of additional training; ended up with some torn muscles in my left quad. The shoe adivce is good. But biking uses different muscles than running, so that will actually be to your detrement. If you can, find an experienced marathon runner to help you train and answer questions. I did a plan that called for runs 3x weekly and it worked well.
Most plans will have you run at most 20 miles at a time. The last 6 of the marathon will be hard because you're probably never run farther and they just plain hurt.
Like was said, listen to your body.
Happy training and good luck.
ETA: You might want to start by competing in a half marathon before the real deal. Also, when you're picking a race to do, take into consideration hills, climate, conditions, etc. Some will even say that they are or aren't good for first-timers.