The only walks I have done recently in the Beacons are to the top of the Sugar Loaf although the last time we abandoned that goal halfway and walked through the forests along the many Forestry Commission Tracks which was nice in its own way.
It's quite a long walk to the top of the Sugar Loaf, the route we take usually starts off with a slog through a steep wood and across some farmer's fields, not sure that there'd be too much opportunity of some snappage whilst you're scrambling up a hill or running from sheep.
The last walk local to Cardiff I went on was from Llantwit Major to Nash Point, that's a nice established coastal path that's not too strenuous, goes from the 'sticky out' (the beach head) bit towards the bottom right of the map, past St Donats castle and on to to Nash Point light house and its huge fog horns and (if the tide is right) back across the rocky beach.
http://maps.google.co.uk/maps?f=q&source=s_q&hl=en&geocode=&q=Llantwit+Major,+UK&sll=53.800651,-4.064941&sspn=15.56051,46.538086&ie=UTF8&ll=51.398081,-3.52026&spn=0.032023,0.090895&t=h&z=14&iwloc=AHowever, my fall back walk is always from Upper Cwmbran across the Mynydd Maen, nice easy stroll through the forest and across the ridge dropping down onto the top of Cwmcarn Forest Drive (you could always get to the Cwmcarn side and just walk Forest Drive?) or push onward to Twmbarlwm and the views out toward the Bristol Channel
I'm not that familiar with many of the popular walks in the Beacons (mostly due to being a lazy bastard and not getting out there) I'm sorry.
Um, there are also a fair few circular walks that start in Penarth if you fancied being a bit closer to Cardiff?