The Gap run would be the most obvious, although it would be a tad rough in parts for the CX bike. This would involve a start from Brecon, over to Talybont (via road), up from Talybont over Brinmore, down onto the Taff Trail (there is another route but this is the simplest), round to the start of The Gap itself, over The Gap and Pen-Y-Fan, back down into Brecon. I can't remember how long it takes me but I'd guess 3hrs is more than ample even leaving time for small flask of tea, flapjack and views.