I think it's a great idea to find a motivator that works for you, if this is it than more power to you (it's would be about 14,000th on my list
I think it's plenty of time, but I would worry that it's too far out/too big without a LOT of interim significant goals (like running a regular marathon).
I've seen some links on the web about training for a marathon, I'd start there realizing you might have to progress more slowly where you are starting from. Maybe start mixing in some cross country as you prepare for a normal marathon like a year away. Once you can do a regular marathon, the jump won't be as significant to the one you are discussing.