I hope you go ahead and do the marathon! I have had a similar experience recently...... I signed up for the April Brighton (UK) marathon, last November 2011, it being my first marathon. As it turned out, due to a chest infection, night shifts and then a leg injury, I only managed a total of three runs throughout January and February. I wondered then whether to cancel and defer to 2013. Luckily, in March, I gave things one last go so long as my leg didn't give way again. I found that despite the time off, the fitness hadn't completely left me. Like the other folks have already said, it's a long distance so I kept all my runs, whatever the distance, to the same pace - no intervals, no speedwork, only whatever hills I came across on runs I did. I kept it to 3 runs a week, 6 miles,10 miles and the last being the long run. I managed to do a 20+ mile run two weeks before the race and then tapered down. I thought the biggest problem would be my feet lasting the distance but on the day, they were fine.
I am SO glad I didn't call things off!! The day itself was a wonderful experience and the occasion will get you through those miles. Whether you run it, walk it, etc, you won't be the only one doing so and you'll find all the support you need to get you though. So go for it, you'll be so happy you did - albeit tired, but happy!