Frus - it is that weekend
Yes, it's crazy, but it's good crazy
I did a marathon last September. I've got really into running, and it's how I lost most of my weight, but even so it was a big ask, and there were times when I wondered whether I really wanted to put myself through it. Just as a pointer of where I was at when the idea popped into my head, I ran a 10k in September 2005 and my second in November 2005. I then decided to try a half, which I trained up for and had planned for February 2006. Before running it (so around this time last year) the marathon idea popped into my head. I ran the half at the end of last February, another half in June and then the marathon in September. I personally think I didn't have enough of a base to do myself justice in the marathon, and I was pretty disappointed with how my training went and the race on the day, but I made it round in one piece, and I enjoyed it enough that I'm currently training for my second (and third!) marathon.
If there is time before registrations close, I'd start the training before committing. If you have to make the decision now, think long and hard about whether you can spare the time (a 3 hour training run really does eat into your day, particularly when you have to spend the rest of the day recovering!), and whether you're committed enough to the goal to stick through it when your legs are tired beyond belief and you can't face running another mile. If you can face that thought, then go for it!