Where abouts in Dartmoor are you going to be?
My favourite off-road loop is this one:
http://www.dartmoor.gov.uk/__data/assets/pdf_file/0011/188876/Burratorcycleleaflet2011.pdf
The descent down to Burrator is a cracker and there's beautiful scenery all round. It's a bit short though if you're after more of an all-day ride.
Another very good longer one is to start at Ivybridge and head up the two moors way, loop around Avon reservoir and head back south through Didworthy, pick up the trails heading SW to rejoin the two moors way (which might test your map reading skills a bit, or at least it did mine). I've only done that one once, it was pretty tiring with some very tough long climbs, but the two moors way section is gorgeous (not very techy, although there's plenty of that on the smaller trails, but epic scenery).
Finally this one is probably my second favourite, on the East side of the moor, and also plenty long enough:
http://www.mbr.co.uk/routes/western-england/dartmoor/dartmoor-devon-killer-loop-256732
Lustleigh cleave has some proper techy stuff with big boulder drop offs (though be prepared for pedal strikes galor!). There are a few short(ish) sharp climbs I've never successfully cleared, including one up to Hunters tor, and another up through Yarner woods to Trendlebere down. The loop around Birch tor is also ace, though I've only managed it once as I'm usually too knackered to add it on!
Do let me know if you want any more info and I'll help if I can. If you want to do the shorter ride around Burrator I may be able to join you if you wanted.