If I ws clever I'd paste you a link to the Oxfam Trailwalker thread over on events, RD ;o)
It's a 100k from Petersfield to Brighton for teams of four. I've done it with Forum teams for three years, this year as an 11th-hour substitute, and have never finished in under 25 hours (the priority is finishing with an intact team rather than getting an individual time). However, big chunks of the route look extremely runnable and I think I've got three other, er, volunteers plus a support crew to have a pop at 15 hours next July.
This year, that time would have made us 10th overall and fastest mixed team, which probably means it's going to be a lot harder than I think it is. Aiming for a sub-4 spring marathon fits very nicely into my training plan, and after the marathon I'll avoid racing anything longer than 10k (unless I get mugged by a Keswick half entry form) and concentrate on distance again.