A good ol' fat boy marathon thread. I love them!
I'm a moderate mileage runner (40+ a week) @ a weight of 175 - 185.
I've run a 18 minute 5K, a 1:27 half and a 3:11 full marathon. I'm running my first 50K this November and feel pretty good about it. In January, I am planning to tackle a 100K but haven't signed up yet.
At 90 miles a week, you should have no problem with the marathon. You will probably experience some fatigue around mile 20 due to low or depleted glycogen, but it shouldn't feel any worse than running on legs that are used to running 90 miles a week (at least physically). With this amount of miles, you should have a few 10+ mile runs through the week that will be good for your LSD.
You don't need to build your base or need to increase your endurance so the only real tips I can give you is:
1) Take a few gels with you and use them around mile 6, 12, and 18.
2) I like to take a cup of water and a cup of sports drink at every other aid station and a cup of water at every aid station. Heavier runners need more of this than the typical 150 lb runner.
3) You don't need S caps at the marathon distance, but you can take a few if you want. They will be more mental than anything at this distance.
4) Taper, taper, taper. Drop back to about 30 miles two to three weeks prior to the race.
5) Enjoy your first one and don't race against the clock. You are in the beginnings of the trial and error stage and please realize this.
6) Carb loading is okay, but don't overdo it.
7) Get a little extra sleep in the taper weeks to recover.
8) Try to get some real food late in the race. Again, this is a mental thing.
Good luck.