I'm afraid even though I've worked in Merthyr I've not ridden around there much, but some of the trail centres around south wales are really good and probably worth a look on the weekends. Have you had a look at the new bikepark wales? That's opening soon and is very close to Merthyr. A friend who lived near Merthyr used to spend a lot of time in Cwmcarn so might also be worth a visit. I'm in Swansea so spend lots of time on the trails in Afan, theres a lot of work going on there at the mo but some new trails opening soon. Brechfa is also very good and a little less busy. I'm sure if you fancied something outside of the trail centres you'll find plenty around Merthyr though. Brecon is good for off road trails and there are quite a few maps available online/from the park visitor centre if you wanted to try that out. The Gap route is quite popular from what I've heard though I've not done it myself. Theres lots to keep you occupied in this area anyway Have fun!
I wouldn't say more, different probably, a little more extreme you must remember a lot off the marked bridleways are unrideable. Access can be tricky too. In my last post forgot to mention the elan valley area excellent riding there too probably on the limit of your 40 mile radius.
