Area is plastered in singletrack, lots and lots of forestry trails as well. Take a look on Google Satellite images
If you stay away from the centres you could ride all day without coming across a soul. Then again the Centres are there to be ridden, whatever floats your boat
Accommodation wise I wouldn't suggest the Valley's but would centre around Brecon itself. Plenty of good accommodation around the area a lot of it catering specifically for those people who like to be outdoors.
This isn't bad, couple of miles from my house, health spa thrown in. Located on the A470 between Merthyr Tydfil and Brecon
Here it is on Bing Maps
http://www.bing.com/maps/#JnE9Lm5hbnQtZ ... IyMDA0Mw==
Just zoom in use Aerial images and click on the OS map option to bring up all the tracks around the area