I'd suggest you turn up at the Ashton Court Golf Kiosk (see map) on a sunday morning at 10.30, maybe post up here to double check first. Local rides out of Ash Ct are usually around 10 miles of muddy singletrack ,taking 2 -3 hours at a mellow pace. That way, you can guage your fitness against others and see how you're doing. Also, you're never far from a short cut home if you're knackered.
Hope this helps, Giler.