Disaster struck for me! Got to 10 miles, then staggered between 10-11 (it took about 1/2 for one mile!). Was intending to stagger to finish, but St. John's guy picked me up as he didn't think I should be going on. He was proably right, not sure I would have made it. 
Little gutted as I did it last year and was absolutely fine with it, finishing in 1:39.
Think I went to fast on some of the early hills as well, which didn't help.
Thanks to all those runners BTW who asked if I was OK (if any read this thread!), was feeling pretty silly.