Sure there are some cheeky routes but when I scoured the map a while back that area seems to be pretty poorly served by rideable RoWs apart from the Camel Trail etc and the woods around Cardinham.
If you don’t mind a bit of a drive Dartmoor is not too far though. You could get to Princetown in about an hour and do the Burrator / widowmaker loop.
Not ridden it but there is also Gawton if you’re into DH:
http://www.gawtongravityhub.co.uk/hsd.php
EDIT: Link to info on Cardinham woods – apparently Blue will be open by Easter. http://www.1sw.org.uk/experience/trail-hubs/north-cornwall/