Thanks guys... I may start the plan next week, but if it isn't any fun, why do it? It's just a hobby after all.
Mace: depends on the venue. If it starts in a big field, you could get there early and do warm up laps until it's time to start, then do the race, then a few cool down laps afterwards? If the start venue doesn't allow you to do this, you could always "keep going" at the end to add on the distance without worrying about a break in your run?
I'm 90% certain I'm not going to do a Spring marathon now. It's just too cold and miserable outside: the idea of running 24km on the way home from work midweek when it's pitch black and minus 1 is not terribly pleasant! The other difficulty is that it could easily be 20-25 degrees in April which isn't ideal if you've done all your training in Arctic conditions. Much more pleasant to train for an Autumn marathon...
The last time I followed P&D (2007) I tended to do the medium long and long runs on hilly-ish routes, but then again I live in Bristol and so flat running routes are few and far between! More seriously, I think doing steady running on the hills does really help with leg strength, especially if (like me) you don't do leg weights.
I agree - turning to XC on the "off" season is really helping my strength, which is a definite weakness in my case!