MikeyJ sounds like you and I were running near enough together for a lot of the early part of the race. I set off with the RW 8.30 pacers from Blue Start as well.
Like you my training had gone very very well and in March I did the Thanet 20 comfortably in 2.47, followed two days later by a PB in the club 10k of 44.56. From there I did three more long runs but admit to not doing very much in the middle due to weather, work, being knackered. At the end of March I went on holiday to Florida and did no training at all, apart from walking around theme parks every day, all day, swimming for about an hour a day and two five mile runs in upper 80's. Arriving back in the UK on the Wednesday before the race and having no sleep for 36 hours probably all combined to my downfall.
So moving back to Sunday I was with the 8.30 pacers through to Rotherhite, this was comfortable... I remember seeing the 9 mile mark and seeing it was 1.21 thinking this was very slow. By mile 11 though my legs were becoming like lead and by Kings Arms (around the corner from my office!) I was reduced to walking for a little bit... This would not have been such a problem because I run walk even the longer distance on approx 1 minute to every 5miles, with the balance at 8mins or below. However on Sunday by the time I got to Tower Bridge I felt like sh%% and by mile 12 decided that the sub 3.45 was out of the question instead I decided to go for sub 4, however by the half way point (1.57) even sub 4 was revised to just finishing the race. From there onwards my goal was simply to make it to 100m before the next mile marker so that I could walk up to the marker before running onto the next marker.
Fortunatly I never suffered as bad as you, but most of the last ten miles were spent with either a stitch or in the last 5/6 miles with pains across my chest which only eased when I reduced down to a walk.
Like you I was not prepared to quite despite feeling like an old toy whos battery was running out while the duracel bunny went hurtling past me again and again.
So it was with every last ounce of energy that I rounded Buckingham Palace to hear the announcer say that anyone who wanted sub 4:30 on their finish photo needed to pick up the pace. I rose to the occasion and staggered ungracefully to complete in 4:29:55, with a chip time of 4:27:06... 45 minutes outside target.