In reply to BMG100:
Any particular area you are interested in, or distance you are happy to travel in the morning? Aberdare country park is, maybe 40 mins from bottom of Pen Y fan at Storey Arms, does good accom for B&B. For access to Eastern Beacons, Abergavenny might be worth looking at. Western Beacons, loads of places such as Llandeilo might be a good place to look which also gives access to some great walking countryside in West Wales.
Not really helpful apart from the country Park sorry - but the walking around Rhondda from there is good too.