RnrActinTech saidCongratulations, you are about to join a population that comprises 10% of all runners!
I agree with a lot of the advice already posted, just wanted to add that doing a 22 miler isn't a bad idea. It gets you past the 20 mile wall and prepares you for the end of the run. For my first three marathons I ran only as far as 20 and for my 4th I ran a 24 miler and that made that last section a lot more manageable.
Also, when you're out running, break up the run into smaller segments, at 20 miles think "I'm running a 10K, I've run many 10Ks before"
Finally, trust your training when you're out. Also, HAVE FUN!!
This is all good advice, but I particularly agree with the above suggestion of running some 22 milers.
In my best race I incorporated into training six long runs over 20 miles and two of those were 22 milers. I ran them slower than race pace and it made a tremendous difference.
The key to a good marathon is in pacing more than anything. Know your limits and run a smart, conservative race. As they say, the marathon is really two races: a 23 miler and a 5K, with the latter often the most grueling physical experience you'll ever endure. It can be alternately humiliating and exalting, depending on how you prepare for it. Train and run smart and you're in for a real thrill. Train and run carelessly and you'll be joining the zombie parade at the end.
Check out McMillan's running calculator for information about training and race pacing (and try to ignore the annoying music he has on the site):
And the best book for your money is Pete Pfitzinger's "Advanced Marathoning." It's extremely well organized with many training programs and full explanations throughout.