I run 5k once a week (most weeks), always off-road. My last run (moderately rough field, approx 300ft elevation) was 27:22. I consider myself of above-average fitness although I'm very much a novice runner. I don't run at "race-pace" though (i.e. I'm not completely spent when I've finished) because I'm not experienced enough yet to pace myself properly if I try to run as fast as I can.
My current goal is to get under 25mins which I think I might be able to achieve on a flatter/smoother field - there is a suitable sports field near where I live so I'm planning to give that a go early on Saturday morning (I'll let you know how I get on!). In my (limited) experience though pacing & technique is as important as fitness if you want to improve your time.