Hi Peter - I'm doing Bristol too and it will be my first 13 miles. I began running in May and tend to run around 6 - 7 miles a few times a week, but my longest run so far is only 10 miles. Like you, I also run about 9 min miles, and while I haven't been injured, I have been too busy (translate: lazy) to train as much lately. I am going to go for it though, and hope for a time under 2 hours, although I suspect I'll be looking more at 2.10 - 2.15. BUT, if I can finish then you can! Good Luck.
Suz