Booktrunk just spotted your advice also. Yes I like that idea. You are right I will be disappointed if I think I could have done better. I suppose the problem I have is that because I have run around 3.48 pace in training up to 18 miles I know I should be feeling ok half way at that pace. What I don't know is how much I'll be suffering at 22 miles!
Might set off somewhere between 3.45 and 3.50 pace!