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I'm with you matty's mum - my training regime and times were similar to yours and I set myself what I thought was a conservative target time of 4 hours 20 minutes. I was cruising nicely for the first 10 miles and then crashed and burned and had to walk and jog to a disappointing 5 hours 13 minutes.
My problems are my legs and knees and I have resolved to work on strengthening them in the coming months. Body Balance classes beckon!