As a regular user and local to the aea ( lived in gilwern and also worked for british waterways/canal trust ) all the canal stretch from pontypool to brecon is good. There are muddy stretches in the forestry sections that have to be kept natural due to habitat and wildlife. Put a set of 32mm cyclocross tyres on and the whole 33ish mile stretch is rideable.
I use it during the winter months as it keeps me off the icy roads and is such an enjoyable ride. Coffee stops at tal y bont, crickhowell and goytre wharf. Plenty of pubs on the way too.
Enjoy