I've ridden the bike off road for a few hours, but currently have 32mm urban tyres on it until my knobblies arrive. I've just been doing sprints, LT and intervals on the road/gym and running/remounts off road since then.
I have nothing but road commuting to compare myself to really, and I most often commute on my Brompton (24 mile round trip)! I certainly keep up with mountain bikers on it, but even going full tilt, I just can't keep up with roadies.
I'm going to have to get some serious off-road practice in I can see.
The race is in Kenilworth, Warwickshire about 60 miles maybe from leominster.