Well, made it back in one piece, great trip but very tiring.
The route turned out to be :
Along the Ystwyth trail to Cwmystwyth
Over the mountain road to Rhayader
Distance 156 miles over 3 days.
Highlights were the reservoirs and dams of the Elan valley and the Ystwyth trail.
Campsites were Wyeside at Rhayader, nice quiet site, only one other tent with a couple doing Lon Las Cymru, and Small Batch in Little Stretton. Lovely site at the foot of the Long Mynd, highly recommend it.
Huge climbs over the mountain road from Pontrhydygroes to Rhayader, very remote and exposed. Big climb out of Church Stretton, another over the Roman Bank over Wenlock Edge.
One observation is that the Sustrans route often gives barely rideable gradients in an attempt to be traffic-free. Several times the route takes you over the top of moutains on ungraded roads where a more rideable quiet road was available.
There is barely any traffic from Aberystwyth to Newtown. I only came across farmers in 4x4s who were exceptionally considerate and patient when i couldn't get out of their way quickly.
Camp sites were pretty scarce so it's wise to plan ahead and not assume there will be one available.