I'm starting with the running/walking thing, too. I've been walking 3+ miles at a time on a regular basis for months, now, and I think I'm ready to start running again. I've also dropped from 210 lbs back down to 180, so I think my knees can take it now.
I bought a new pair of running shoes yesterday, so there's no backing out now. I don't know about the marathon thing, but I'd like to get to the point where I can run 3 miles 3 times a week, then see if I feel like going for more.