To follow up on Martin's comments Steve
olympic distance: 1.5K swim, 40K bike, 10K run.
half ironman: 1.2 mile swim, 56 mile bike, 13.1 mile run.
ironman: 2.4 mile swim, 112 mile bike, 26.2 mie run.
ultraman: 6.2 mile (10 K) open ocean swim, 261.4 mile (421 K) cross-country bike ride, 52.4 mile (84K) ultra-marathon run (at least they give you 3 days for this last one!!!!!!!! Check it out at:
http://www.ultramanworlds.com/
DavidB, I have to say that although I'm not convinced of the need to do high (running) mileage for marathon success I do think that the long run is one of the cornerstones for doing well over 26.2 miles. IMHO I think you can get away with dropping easier runs in favour of cycling (or some other aerobic/strenghtening exercise) but I think that speedwork, tempo runs and the long run are crucial. Of course all this is entirely subjective and may be entirely psychological!!!
All the best
mm