There is a bothy that you could stay at for free!!!! And would easily breakup you're day up for multi days. Backpacking around the black mountains.
Plus if you try the nutalls book volume 1 Wales. That has some nice routes in, but at the same time depending on you're speed of walking. Are quite long and given the time of year when the winter nights become less lighter than summer months. You may like something of the 10 miles area. But here are two good 13 half-ish - 15miles routes if you like starting north from Crickhowell there a small car park. But be warned it dose fill up really early. (we got here for 7:30am, and it was full), the tops you take in are Pen Cerrig- calch, Pen Allt-mawr, Pen Twyn Glas, Mynydd Llysiau, Waun Fach, Pen y Gadair Fawr and Pen Twyn Mawr ascent in total is 2850 feet. The next we took in these lovely mountains as well Black Mountain south top, Black Mountain, Twmpa, Rhos Dirion and Chwarel y fan, ascent 2400ft, again be warned there a car park which fills up limited car parking again..... Which is a groovy two days of the Black mountains. The only difference is that the routes I put up are the other way around in which I did them........ Plus if you did it the way I did, than you can see where you walked the day before if good weather, and it's amazing really is so should be awesomely fantastic at Christmas!!! Pen Y Fan does look interesting as will.
A few good links at the bottom.
http://www.go4awalk.com/home/http://www.blackmountainholidays.co.uk/