AHHH, marathons! What is the current shape you are in? What is your allocated time committment you are willing to spend to train? Once you have these answered, you will be able to find a suitable training to follow. I ran my first full marathon 6 weeks after my son was born with absolutely no training. I will never do that again. I was lured by a race in San Francisco geared towards women with a prize above all others at the finish line - a Tiffany and Co. necklace. Now, I live at 10000 ft elevation and hiked all over the mountains after my son was born, with him hugged close to me by my bjorn and thought that was good enough to get me thru 26.2 miles. I finished, although my body was very very angry at me. I waddled like a penguin rather than walked like a human for the 2 weeks to follow the race. Since then, I vowed I would never do a "full" marathon again, that half marathons were just as fun and less of a torture on my body. That is until this year! This year I have signed up for the Marathon Du Medoc in France and I am going to actually train for it. Now, I feel like I am in decent shape and I have 6 months to prepare. I have a support group of 13 women following me to France to do this race with me. So that being said, we found a great 26 week training program. If you do not have 6 months, there are several wonderful training plans out there that last range from 12 to 16 weeks. Any of these are good tools to get your body ready for the race.
I wish you the best of luck and the hardest part I have found so far, is committing yourself to follow thru with the workouts. Life tends to get in your way and somehow you can always find something you should be doing rather than running, but set aside that time and do it!
One last thing, find an amazing playlist to have on your MP3 player. There is always one song that makes you smile and helps to pick up your pace and spirits!