Carl,
My best marathon time is 2-52, but my 1st IM marathon (Longest Day) was just under 4 hours (overall time of 11-05). I have since improved to 3-31 for the run as part of an overall time of 10-31, but this was achieved by increasing the cycling training, not the running - I think I actually ran fewer miles that year. The point is that it doesn't matter how good a runner you are if you are totally wrecked when you get off the bike, you need to train properly on the bike to give yourself a chance on the run.
Martin.