A couple of fabulous walks 10 and 8 miles respectively, with photos of what you see on the way, taken from....
http://heritageofwalesnews.blogspot.co.uk/
The Source of the Usk
http://www.rcahmw.gov.uk/media/278.pdf
THE FAN LLIA AND FAN DRINGARTH WALk
http://www.rcahmw.gov.uk/media/276.pdf