I used to run there a lot, a favourite from Hafren car park to source of Severn, straight over and down to Hengwm and then down the Hengwm valley to Maesnant. Follow a path up beside the Maesnant direct to Pumlumon trig. Back along the ridge to Arwystli and make my way back however I felt.
Loved the area and camped it a few times, true peace and quiet. Plus midges!