Those who walk on the north side of the Embalse de Yesa should not count on much in the way of waymarking. In September 2009, it consisted very much of walking along the side of the carretera-- I found one splendid path by the side of the lake which cut off a few kilometres by means of a discussion with a shepherd who, leaning on his crozier with the sheep massed around us, used a Google Earth app on his mobile telephone, and drew me a map to find it the detour. With the exception of one gasolinera, there are no water sources until you are about 80% of the way to Leyre, where there are several campgrounds. I do remark on the hot springs at Tiermas (km 366 if I recall correctly) where I joined a few dozen Spaniards in a cheerful mud bath and was well exfoliated.
At the end of it all, I was too tired to walk up the extra few km from Yesa to the monastery, and stayed at a hostal in the pueblo. There were 2 French cyclists there and I passed a German pilgrim, but that was all between Berdun and Yesa.
That having been said, Priscillian is quite correct in that the scenery is spectacular. I also recommend the castle and museum at Javier, the birthplace of Saint Francis Xavier. People in Yesa and Javier were friendly and helpful.