“Arthur’s Stone is an atmospheric Neolithic burial chamber made of great stone slabs, set in the hills above Herefordshire’s Golden Valley. “ [http://www.english-heritage.org.uk/visit/places/arthurs-stone]
This loop starts from Talgarth, through Hay-on-Wye, towards Dorstone where you hit a very testing hill shortly followed by Arthurs Stone. Dropping back down to Bredwardine, Eardisley, quick Coffee stop in Whitney and head back to Talgarth on the A438.
A decent 40 mile ride taking in the countryside of Wales and Herefordshire.