Marathon training

I've signed up to train for a marathon, well actually a half marathon (I'm not crazy).My local YMCA had a flyer that stated if you want to train for a half marathon show up at 6:30am this past Saturday. So, I showed up along with about 15 other folks, all a little unsure of what exactly we were getting ourselves into that morning. The trainer showed up and asked us which training were we here for (marathon or half) and said we would have 14 weeks to meet this goal. What?!? She actually is going to have us compete in the Houston Marathon. Gulp, I thought we were just training. Several other folks obviously were thinking the same thing because I saw the same look of worry cross their faces as well. However, our trainer has absolute confidence in us and believes that all of will be able to complete our training and the marathon. We shall see. We have many early morning runs scheduled for the next few months, with them getting earlier and earlier as the distances get longer and longer. I've going to give it my best shot, since this is one of my "life goals" and it would be a great thing for me to be able to accomplish it. Let's just hope my bad knees and heels can hold up under the pressure.