Can you do any sort of intervals on the circuit? For example, all out between telephone poles? From one stop to another (or driveway)? The intensity will really help. If it is hilly, you could drill all the ups, and recover on the way down.

And 30 mins is likely not quite enough - perhaps 2 loops of the circuit? My typical cross workout is between 1 and 1.5 hours (sometimes 2). I found the following here:

Training Week (assumes the previous weekend was a race weekend):
Mon: recovery day – off the bike – focus on SSHNs of Recovery
Tues: 1hr easy ride w/ 3-5 seated 10 second sprints or easy 15-20min jog. If you haven’t run in a while, you need to ease into the jogging slowly to prevent excessive soreness – do a walk-jog instead 2mins jog, 2mins walk, etc.
Wed: High Intensity Day: 4x10min LT efforts slightly below race pace. Rest for 5 mins b/t each effort.
Thurs: Sub Threshold Day – 1.5hr ride w/ 60mins of steady “tempo” pace – you should be above conversational pace but not labored breathing.
Fri: recovery day – easy 15-20 min jog
Sat: High Intensity Day – Hard Group Ride or 3x15min LT intervals just below race pace
Sun: Sub-Threshold Day – 2hr ride w/ 60mins of steady tempo and 30mins of skillwork

I race both Saturday and Sunday almost all season, so wouldn't do those prescribed workouts necessarily. I also don't run, except in races (hmm - maybe I could be faster if I did some running? ). On the Tues or Thurs night, I would switch a bit and do some skills - dismounts, corners, sand, mud, etc.

My intensity day is definitely Weds, and the workout here is a good one. This should at least get you going, and a dirt road circuit sounds perfect!

SheFly