Burrator Reservoir is about 10 miles outside of Plymouth and is in fact on the edge of Dartmoor, the views you will find when you get the there will make the trip well worth it!
The loop around the reservoir is 4 miles and is relatively flat. The route takes you along some quite narrow country lanes, it is likely you will encounter some traffic but there is plenty of room to let them past. You also need to watch out for horse riders and cyclists because the route is very popular for them as well.
If you want to get off road and add on some extra distance, there are plenty of entrances into the plantation on the banks of the reservoir - watch out though there are some boggy bits!
After your run you can refuel at the Burrator Inn (1 mile away) which serves good food - especially their Sunday lunch.