Keep on believing man. You have had one or two difficult things in life to deal with in the last several years, but it appears time has not withered you, but made the man stronger.
You ARE becoming a rolemodel, espeically to the more maturer person, becasue you've proved so many times that aging doesn't mean you accepting it and not trying to strive for more, when more isn't there, because it is, you've strived and you've earned the rewards.
I know you will make a darn good fist of training for that marathon, even if it means possibly having to pull out, (hopefully not so though), but the main thing is you wanted to do this and the person you are now, is a person that would not cave in after a week and act like it's beyond their capabilities, but someone who will try within the boundaries of good physical reasoning, to make this want a reality and a done deal.
You have shown people you have more human worth and value, by realising it yourself. You've doen great things in the past year and a bit and I know a marathon is now something you ARE ready for mentally and emotionally and I know with your old forces background, you'll have the grit and will to commit to the training process and see it through and I wish you a lot of luck trying
Keep us posted and keep on trucking fella, because you've done a great job of turning your life around and you CAN be proud of yourself and if a certain Lady was here now she'd be proud of you for what you've done and I think you've probably started to live in a way, she'd always hoped you would and would approve of, which is to grow and burgeon off the back of difficult circumstances, not wilt and become a lesser man, which would have been an easier thing to do.
I think you've definitely done her proud big guy, so hang in there and if not necessarily for yourself, then do this Marathon for her and let her be the inspiration for a potentially crowning achievement in your life.
I know though, however and why ever you chose to do it, you'll do it well, because you've become a man who doesn't do things by halves