Okay, here’s the “straight dope” from one who walked it in 2019 but more importantly just drove it TODAY!!!! You cannot get fresher news than this!
Do NOT walk the north side of the embalse (and I say that as a hiker who routinely picks odd and difficult routes to explore). What was not great four years ago is now terribly tragic.
There are ZERO services on that side of the water between Berdún and Leyre unless you detour off the path 4km roundtrip to Sigüés. You will be walking on portions of the old highway with car and truck traffic which gets even worse when you reach the area where the new highway is incomplete and all the area traffic shares the road with you. And when I say road, I mean it because there is often no shoulder and you’ll find yourself walking in the traffic lane most of the time, especially across the multitude of bridges you’ll have to cross. There is little shade and absolutely no potable water sources.
While the Tiermas baths are interesting, they rarely appear above the water line and usually only when it’s so hot and dry that you’d be crazy to try to hike there. The abandoned mountaintop towns of Esco and Tiermas are actually quite cool, but Ruesta on the southern variant is similar and actually has an albergue and bar. And while the Leyre monastery IS interesting, your stay there will be very expensive unless you secure a cell in the monastery itself. You can easily day hike there and back from Yesa and the pilgrim albergue there was quite comfortable.
If you take the southern route, you can route yourself from Ruesta through Javier to Yesa, seeing a brilliant and historic castle enroute. From Yesa, pass through Liedena and take the path through Foz de Lumbier. You’ll have to stay the night in the town of Lumbier, but the only hotel there will give you a discount as a pilgrim.
Finally, both the Monreal and Sanguesa albergues are currently closed due to management issues and will not be opening up for the foreseeable future. A true shame as each serves a vital link in moving pilgrims through the Aragon Valley.
The attached photos show the typical path and my wife literally giving up after 8hrs of hot sun and no water with no end in sight (she rallied and made it to our stop in Artieda as we were reverse-walking it this time)