First, let me say that I am not trying to be unsupportive of anyone training for a marathon right now. It is an ambitious goal. Is it a realistic/ideal goal for everyone? I don't believe that it is.
Tonight there was a program called "Marathon Challenge" on PBS. Very good message about how running throughout one's life has great health benefits, but I felt like they really glossed over the question of whether running a marathon is really appropriate for everyone. A 300-pound former linebacker isn't the traditional physique of a runner for good reason. In addition to the pounding on joints, the ratio of slow-twitch vs fast-twitch muscles isn't ideal.
To be fair, after running four marathons in the past eight years and with a half marathon PR somewhere around 1:35, I continue to consider whether running marathons is even right for me. My current conclusion is that I should only try another marathon if I can do it "right" which means, for me, not just training well, but that I can do it fast. It is a little bit of pride and a good dose of avoiding too much wear-and-tear.
Marathons have become very mainstream, but didn't the very first Greek to run a marathon distance, keel over and die at his destination? I fear that the popularization of marathons is short-changing shorter distances as a really good goal to get people exercising and even competing.
On the other side, I also do NOT consider the marathon to be the ultimate test in fitness - as mentioned in an earlier commercial for aforementioned TV program. A marathon is just an entry point into the world of crazy-stupid endurance events ;)