Hello Andy

I'm currently dahn sarf in London, and have only run in a handful of races, but for what it's worth I'd say you've got nothing to worry about. If you're enjoying running in beautiful surroundings and/or knee deep in muck then you'll have a ball fell running. My main apprehension before the my first event was whether I could keep up with the pack, but what I've found is that every race attracts such a wide range of runners that there will always be someone you can keep up with.
And everyone I've met has been friendly and encouraging.

You'll be fine mate