How about following the famous Coelbren Roman Road, starting from the Mountain Centre above Libanus, and heading towards Coelbren.
The distance is just about right for a long weekend, and the route takes in some great parts of the Beacons National Park, that not that many people bother to explore.
The route, is almost entierly flat, so i am afraid there are little scrambling opportunities, but i did the walk back in 1995, and enjoyed every bit of it.
The views are incredible, its well worth checking out, but most of it goes through very remote country, so the weather has to be fairly favourable, as you will be in the teeth of it, with nowhere to take shelter.
The section from the Mountain Centre, where it starts in the Pen Y fan and Forest Fawr direction, is not the place to go in driving rain, i learnt to my cost, when traversing various sections of it, your clothing has got to be in tip top shape.
The Road from Brecon's Y Gaer fort, is not easy to follow, the archaeologists know the course it took, but from the Mountain Centre, its direction is never in doubt, so this is where most walkers doing the route will start.
Maybe more of a spring summer adventure, or a winter traverse for someone with the experience and knowledge to navigate in possibly hostile conditions.