lots of places to ride down there –
back in December i rode the a route south of Postbridge South which is quite nice – gets a bit hard down the bottom as the track disappears and you have to do a bit of climbing down a hill (or cycling if you are confident enough – by the hairpin on the map which is south of Babeny)
IM sure some of the more local riders can give more advice!
If you go south out of Princetown there is a nice trail all the way down to sheeps tor – from there you may be able to navigate North west over to the viaduct and then back round to princetown – ive done these on walks before and they should be viable on a mountain bike. East of that track on the south moor is very wet most of the year around so wouldnt see it that good for a bike.