Blakester, I'm not sure there is an average time really as all of these type of events are all so different. It's best not to worry about timings and to just go out and enjoy the experience.
Muv - No worries, I think your goal of 'just finishing' is similar to most of us. It's difficult to set a target time as there are so many variables.
GU - I think I'm in the same boat, The last two nights I've been woken up early and thinking that I've got the start of a really nasty cold, thankfully it doesn't materialise when I wake up properly. I went for a gentle run yesterday and my knee seemed unhappy about it at the start. I've not had any trouble sleeping as I have a baby at home. When you've got one of them, when you attempt to sleep, it's easy. I think it's probably natural to have doubts as it's a long way, but you'll be alright. I'd recommend setting off gently and don't push yourself if you're falling behind time. I've never done this distance before so my advice may be completely wrong but it's worked for about 15 runs of 30-45 miles.
EGG - I like that idea, I'm very much holding off at the moment as the weather is so changeable. If come Friday afternoon at registration the forecast is still unfavourable then I will go for the full waterproof but not before then.