To be fair, whilst I'm a lot more into that sort of thing than Munqe-chick, and could actually race one properly if I put my mind to it, I still reckon a marathon is pretty boring. We've suggested plenty of more interesting things though (and good call on the fell races and LDW challenges).
Not sure how you can even mention OMM in the same context, crazy-legs! Nothing at all like it, with people following lots of different routes, taking home their own rubbish and not waking people up in the small hours. Not to mention that 99% of OMM competitors are more capable of looking after themselves in the hills than 99% of 3 peakers.
A mountain marathon is of course another good idea for a challenge - or if you want to stay with biking there is always Polaris.