I have run one and walked one. I ran one in 2002, and I walked the Portland Marathon in 2005 with my mother. I was about to turn 40, and she was about to turn 62. It was her idea, and she chose Portland because it is quite walker-friendly, and the course is open for 8 hours, which is plenty of time for walkers to finish.
I'm sure we found an online training guide. Walking one is quite different, IMHO, from running one. For one thing, it's a lot harder on your feet -- one, because you are on them so much longer, and two, because they just move in your shoes and interact with the road on each footstrike differently than when you are running. In our early training, we both had LOTS of blisters. I swear by Sportslick (
www.sportslick.com) as the best lubricant, but I know there are other good ones out there. It also takes more water and fuel because of the time it takes, but, since we were walking, I found I could actually eat a PowerBar or something during the event, rather than just Gu, which is what I used when I ran a marathon.
It was a little challenging psychologically when I reached the point in time where I had finished the marathon that I ran, but I still had hours to go walking.
I don't think it should be underestimated just because you are walking and not running -- it's still very challenging. And the training can get long and little tedious. I remember walking for hours in my neighborhood with my Camelbak on -- I felt like a real goob.
My mom did get really tired in the last couple of miles, but I kept pushing her -- I didn't want to leave her behind. We finished in under 7 hours, which was our goal.
All in all, it was a fantastic experience. I agree 100% with andtckrtoo that there is nothing so empowering as completing an endurance event! Good luck!