This route is similar to the MBR route mentioned above, but also considerably longer than 2-3 hours http://www.geograph.org.uk/geotrips/1567
The descent from Y Clogydd back into Llangynog is worthwhile in itself, and you could certainly do that section of the route from the valley to the south, effectively by doing a circuit of the hill called Y Garn (or Glan Hafon, depending on the scale of map you look at).
Alternatively, from Llangynog, take the lane up Cwm Rhiwarth, which turns into a (tough) off road climb up the head of the valley. Either of two natural bridleways will then bring you back, generally southwards, into the valley of Cwm Pennant. Haven’t done that for some years, but I seem to recall it was mostly easy to follow on the ground. It would be useful to have a map in your pocket/pack, though. The area of the southern Berwyns is mostly sheep-farming moorland, so open country once you’re above the valleys.
From Penybontfawr, there’s a number of legit tracks to the south, but again you’d probably need a map to hand in order to be able to string something together. This area (I think) is a bit more enclosed in terms of the farming, not so much open moor.
None of the above is really particularly steep or tech, to be honest…but I don’t seek that out particularly.