Usk Reservoir: A cricular dog walk
Usk Reservoir is in a remote area surrounded by forest and moors and watched over by Mynydd Du, otherwise known as 'the Black Mountain' (there being more than one Black Mountain, this can be confusing).
There is a 6 mile circular forest track on which you can walk or cycle on all around the reservoir. I have on occasion used a Segway which permitted me to explore all the tracks branching off as well. This gives dogs a good run, though you will have to come back along a small section of the road a bit at the end (going anti-clockwise).
If you join the road too early, you may have come out on the more major road that you turned off to get to Usk Reservoir. Either way, whichever road you come to, you turn left on to it, so will circle round to your car eventually.