I'm running my first one this January.  Furthest I've run at a single time was about 12 miles (did that once a week for several months while at Camp Schwab, Okinawa), and I commonly run up to 5-8 miles now.

My goal is to finish without walking.

My wife and I are currently training for our first Marathon. We did a half-marathon earlier this year and it was tough, so we decided to get some help training for the full marathon. We are using the Atlanta Track Club Training Program. It's a solid program that starts you off slowly and builds up the miles. IIRC you don't live in ATL, so I would suggest that you ask around at your local running store. They should be able to direct you to a training program. Running with like minded people definitely helps keep you motivated.

Accountant

Motivation ain't my problem, but a training program always helps.  I ran cross country in junior high and high school, and ran like a fiend when I was in the Marine Corps and on the SWAT Team.

Which marathon?

I hired a coach to help me prepare for Marine Corps marathon in 2003. I finished in 04:02 . I would have broken 4:00 hours but bonked at mile 21.
Walked and ate some Famous Amos chocolate chips cookies and drank water to recover.

Check out
http://www.crossfitendurance.com/
4 Months to a 4 Hour marathon

You can check out the Atlanta Track club and running store like Fleet Feet Johns Creek and Big Peach Running Company for running coaches and marathon training groups.

Monk

Cms,

Google several different training programs, but basically you will have a weekly long runs ramping up over the next several months. I would add, don't get too hung up on running the whole way, there are several studies that show you will have a better overall finish time by walking/running. Also less chance of injury. Other advice that may sound odd, lube, advil, ice, massages, all will help!  You will have a hardbtime walking for several days after, but feel a great sense of accomplishment. Good luck,

Call Crispen, he does iron man triathlons.

Training, hydration, nutritionals.   You'll have the most fun and avoid injury if you have a bonified training plan using a calender and some precision in your training runs.  The hydration and nutritional details can be ironed out on your longer training runs and can't be taken lightly.  Big Peach Running company has several locations around ATL and they specialize in running advocacy - i'd use their experience to help if I were you.

I've done a few marathons and a 50k, personally I think that nutrition and hydration before and during the race are more important than your running regime-

I did one run over 14 miles before my first and its rare for me to get the time to run that long anyways. Just figure out what works for you because little things extrapolated over that distance will break your feet/blister/etc

I've trained for and ran the Honolulu Marathon twice in '93' and '94.'   I bought a book called "How to train and run your first marathon" which also included a running log.  The training consisted of one long run a week with sprints, running up hill and shorter runs during the week.  My longest training run before the marathon was 18 miles and was also one week before the actual race.  Concentrate on running for a set amount of time and not a set amount of miles and listen to your body to avoid injury.  The day of the race was exciting and made the miles go by a little faster.