Munge Chick, the Taff Trail doesn’t pass through any estates in Merthyr Tydfil and it certainly isn’t Tarmac all the way to Brecon either!
To answer Catfoods question you could ride your 3 gear bike on the Merthyr to Brecon section, the Taff Trail turns from tarmac to gravel fire road around Pontsticill Reservoir with one or two slightly steeper sections and further beyond that follows the former railway line (gravel) along the length of Tal y Bont Reservoir. Past Tal Y Bont it follows minor roads/lanes and/or the canal towpath to Brecon.
There is a bike bus that runs from Cardiff-Merthyr-Brecon but I think it only runs from May to the end of September.
Brecon Beacon Bike Bus
There are several good offroad routes from Merthyr to Brecon such as the Gap route mentioned above or the Brinore Tramroad,the following website will give you a general idea of what’s in the area. Mountain Biking Brecon Beacons[/url]
Hope this helps.