@dean richardson 7 - Eddie Izzard's times improved as he went.
Eddie Izzard didn't start off by doing his top paced marathon though.
So in Dean's example, the OP wouldn't be doing 5:30,7,8, he'd probably be looking at starting at 7 or 8...
Interesting point. I think this is the key to it all. Start the week gently and build up (if I feel able) but I'll get into that when I make my final decision and start training.
@dean At the height of my marathon training I was around the 50 mile per week marker. I'd need to up my game to do this. I'll be starting almost from scratch again and building slowly to marathon ability within 6 months and going from there.