Afan is awesome fun. There are 2 trail centres, bike hire is available at either.
Old Afan Trail Centre
Penhydd - 17km - Classic trail, fairly basic but still good fun
The Wall - 23km - Dodgy fire road ascent but an amazing descent
W2 - 44km - You can start this at either trail centre and use the other as a half way lunch stop, I would personally start at the Old Afan Trail centre and use the newer Glyncorrwg Trail Centre for lunch because it has nicer facilities. W2 is The Wall and Whites Level linked together by use of a fire road.
New Glyncorrwg Trail Centre
Whites Level - 17km - Tough 6km singletrack uphill at the start with a fun descent and a black section you can session
Skyline - 47km - Never ridden this, its meant to have a lot of fire road but the final descent is supposed to be good
If you plan to do an all day ride I would highly recommend the W2 trail. For a half day ride any of the trails are good but my favourite is Whites Level.