You pretty much HAVE TO do the full mileage at least once. You need a baseline as far as pacing is concerned and you need to condition your connective tissue to have the crap beaten out of it for over 4 consecutive hours. It's a whole different ballgame but best of luck to ya.
I have only done one marathon (Equinox off road marathon back home in Alaska) and I trained for a solid 2 months (constant running) for it and it kicked my butt bad!!