Agree with that, but to add a bit more length and climbing, turn left over the dam bridge after the new mtb trails and follow the cycle path for a further few miles.
Cross the road, then follow the gravel path uphill through the trees before turning right before the dam at Pen y garreg and head for the black cottage climb, stick to the left and it’s all climb able(although a tough one)
Once on the top head left on the Well used grassy track towards esgair penygarreg before heading down to the road.
Turn left and take the road up to Penrhiw wen and follow the track which picks up the golf links descent and back into Rhayader
Don’t underestimate the climbing, can easily hit 1400m climbing in sub 40k