We walked this route as the first part of a very long combination of a number of caminos, starting in Lourdes and ending in Montserrat. The walk from Lourdes to Oleron to Jaca is not at all difficult, rolling hills and valleys to Oleron, a steady uphill three-day walk to the Somport pass and then downhill all the way to Jaca. Here's the itinerary we followed:
Start in Lourdes
Bertharram 17 km
Arudy 23.4km
Oloron Ste Marie 23.5 km
Sarrance 22 km
Urdos 25 km
Canfranc Estacion 22km
Jaca 24 km
Tips: The meals enroute were delightful. After all, this is France. In Oleron, don't miss the cathedral portal and be sure to enjoy some galotte and crepes. Our stay in the monastery in Sarrance was unforgettable. The walk from Urdos through the pass at Somport was a sharp contrast to the the challenging, up-and-over from SJPDP to Roncevalles. Rather than an up and over, you will be walking next to mountain streams with towering peaks above you.
We enjoyed Jaca so much we spent two nights. And if you have time, take the 0830 bus up to San Juan de la Pena. The 9th century remains of the monastery down the hill a bit is a jewel. You can then walk downhill back to Jaca...or onward as we did. Bon chemin!