I don't know if this is the right forum for a post-mortem, but it may be helpful to those yet to run, or me if (when) I do it again.
I ran with the run-walk group up to Tower Bridge, on schedule for about 5.20, comfortable, waving to the crowd and high-fiving the kids, then my quads started to burn. I realised I would have to slow down a bit and started to drop back. At 15 miles they seized up completely. I kept going alternating walking with a sort of jog-shuffle, and eventually finished in 6.03. By this time I was also very cold and wet!
So what went wrong? I followed the get-you-round schedule, at least all the long runs, and managed 20 miles in the 4 hour one. I missed some of the weekday shorter ones, but did some cycling and gym work. I was also unable to carbo-load properly.
Obviously I should have trained more, and started more slowly, but is there anything else i could have done differently? (I am male, 50 and have been jogging for about 7 years)