I'm kind of the opposite. I run Marathons and try to cycle to cross train so I don't kill my body. I've found that I truly enjoy the cycling - maybe more than the running, hence my draw to triathlon. All of the advice listed above is great. The thing that struck me as the most true is that not many marathon training programs factor in cross training in other sports - they are all geared towards running, and running only. The first thing I'd do is try to decide if you want to race your marathon or complete your marathon. StanSeven was right when he said you can train on less, but you'll be struggling on the course. Based on how much you ride, I have to assume that you are in pretty good shape. Riding shape is different from running shape, though. I can run a marathon an hour faster than my brother, but he can ride longer and faster than I can - just shows where we spend our time training...
He trained for his last marathon using a program from F.I.R.S.T., and liked it. It focuses on 3 key workouts each week, and gives a lot of flexibility in your other workouts. He was able to contiue working the bike, spend a little time in the pool, and prepare for a marathon with about the same number of training hours I put in on just running. The link is below:
I might suggest trying a half marathon first (as part of your build up training). It's a good test and prepares you nicely for the longer distance. To a first time marathoner, it's more about survival than racing, but the more you do them, the easier they get, and the more they become like racing. Running a half is like running a mini-marathon.
Good luck, and maybe this will help you... Let us know if you have any other questions.