think towpaths, old railway lines etc as opposed to BPW!
Based on this, I’d echo Tinners’ suggestion. The seafront cycle path from Mumbles to Swansea Marina. About 12 miles, there and back. Very popular but apart from the busiest of sunny days it will be rideable. I use it a lot – most cyclists in Swansea do, I’d guess. It has the advantage of lots of cafes, etc, and is a nice ride.
You can divert up Clyne valley cyclepath, which leads up onto Gower or can extend down as far as Pembrey, all on cyclepaths, tarmac or gravel, with only a short stretch on quiet roads.
You can also head up the Swansea valley, again mostly on cycle paths, much quieter this time with some nice views as you get up nearer the mountains.
Check out NCN43. You can probably do over 150km, starting in Mumbles and barely touching a road with those routes above.
If you stay on Gower, the roads on the north side tend to be much quieter. The main south Gower road can be horrendously busy at times. But there is plenty of nice riding.
In my opinion there really is no need to travel as far as Brechfa, Afan, especially Cardiff – the riding around Gower and Swansea is nice and scenic and there is loads of it.
Gower is beautiful. Enjoy. 🙂