[Note: the T4 only runs every 2 hours in the morning, and the 8.41 would mean we arrive at the lunch pub long before it opens. Hence the late start, which on the first day may make sense anyway, giving time to stock up on supplies for the picnic lunch walks and/or to get bearings in town.]
This strenuous route explores the easterly part of the Fforest Fawr area of the Brecon Beacons, starting from a high pass between the Central Beacons ridge and the imposing Fan Fawr.
Often ignored by walkers due to its vicinity to Corn Du and Pen y Fan, Fan Fawr is an impressive hulk of a mountain, after a steep ascent providing for fantastic views across the Fforest Fawr out to Black Mountain and across to the Central Beacons. You then descend along the broad and gentle flank of it, eventually along the watershed between the Hepste and Mellte catchment areas, two of the three main rivers of the Waterfall Country just to the south. Stretches of this descent are fundamentally pathless and with some watery although not boggy areas, but in good weather the onwards route is not difficult to discern. In bad weather though, good navigation skills are essential!
You cross over from sandstone terrain into the limestone upland of the Mynydd y Garn with its shake holes and limestone outcrops, and follow the limestone rockface of the Gwaun Cefnygarreg atop the Mellte valley to lunch in Ystradfellte. Rising out of the valley, you cross another limestone upland and ascend the gentle ridge of Fan Llia and Fan Dringarth, after which several shorter options to finish are described.
But the main route ascends Fan Frynych, a glorious viewpoint overlooking the Usk Valley, followed by a long, scenic descent into the valley. Lastly, you cross the grassy common-with-views of the Mynydd Illtud as well as pass the National Park Visitor Centre en route to Libanus with its pub and bus stop.
A (map-led) shortcut greatly reduces the effort. An extension via Twyn y Gaer (hill) is described.