Simon I think the route they were on about is this.
Start from the High Moorland visitors car park at Princetown. Go back to the mini roundabout & straight across between the two pubs. The trail takes you uphill through 2 x gates & onto the moor along a very nice & well defined bridleway. Go past South Hessary Tor on your left & just follow the trail which has a number of granite water bars to roll/crash/jump over.
I would suggest for a first time on the route that you stick to the trail & don't turn off. After a few miles you will eventually go down a long rocky section past the scout hut at Ditsworthy on your left & then onto a tarmac road. At the end of the road turn right cross the cattle grid & down the hill to Sheepstor village. Go through the village do not turn off, this will lead you to Burrator reservoir dam.
At the end of the dam turn right & go on for about 200m, take a left turn off the road which is a tight switchback & go up the hill. Follow the trail which has another 180 degree right hand switchback which takes you on to the old Yelverton to Princetown railway line. Keep on the trail through the woods & the various gates until you get to the end where there is a stile to get your bike over. Cross over the B3212 – be careful crossing the road the cars travel quite fast along there – anyway continue on the singletrack trail through another series of gates. This leads onto another part of the disused railway line & will eventually take you back to the car park at Princetown.
The total distance is about 14 miles but the ride back up the old disused railway line about 4 miles is a gradual gradient & nothing too steep. Can I recommend that you buy the OS map OL28 which covers Dartmoor, that way if you get lost you should be able to navigate your way to safety.
You can of course ride this route in the reverse direction but I'd suggest you go don't this time out.
Have a nice ride.