There are a bunch of training plans you can find online and they all share one thing in common: start out easy and work your distance up, then back off a month before the run. Make it to 16 or even 18 miles as your longest training run, give yourself at least a couple weeks to recover from it before running an actual marathon.
Identify your pace! There's a ton of rules of thumb out there based on half marathon times. I looked at my finishing time in a half marathon 2 weeks before the full, then used that to determine the percentile I was in and how it corresponded to the full marathon results. Saw that it wasn't crazy to do a sub 4 hour, so I set my pace accordingly. I used both a running watch, and in general kept with the pace flagger.
Final word of wisdom: don't do what I do and train in only 5 weeks! It's way harder on your body that way, even if you are in good cradio shape (I bike alot). Your muscles need to build up to stabilize your knees.