The Doethie Valley is rumoured to be the longest piece of legal natural singletrack in the UK. I'm not sure if this is true or not, but it is certainly long (about 8km) and certainly single (never wider then 30cm). It feels very remote and quite special, but it is not really technical, just demanding of concentration!
I think there's only one sensible way to ride the valley in an overall loop of 25km. It is often labelled as being a long and committing route but I don't think it is particularly more so than most natural rides. You start the ride in the Llyn Brianne car park and ride anticlockwise around the reservoir. The track rises above the reservoir, leaves the trees and eventually gets to the Most Remote Chapel in Wales at Soar Mynydd (according to the Singletrack article). From here you take the steep doubletrack west until you almost cross the Doethie Fawr river. This is the start of the singletrack which you follow all the way to Troed-rhiw-ruddren then you take the shortest route back to the start.
For accommodation the campsite at Rhandirmwyn is very pleasant but expensive as its a Camping and Caravanning one. The pub and cafe in the village are also excellent
Rating: 3, "Good riding, worth travelling to".
Added: 2014-03-15 , Last updated: 2014-03-15