Last year I posted a question about getting from the Voie de Vézelay to Camino del Norte. Glenshiro replied but I rejected his advice.
I’ll tell you what I did and comment on the utility.
I picked the shortest route from Saint-Palais to Irun.
Day 1. To Saint-Esteban. I had already walked from Osserain when I headed off trail from Saint-Palais. I took the GoogleMap route, initially on the D8 then up a road called Archiriborroa. I followed up to the top of the road where it stopped and a kind of track took over, really only a gap in the bushes, in long wet grass. Then past a weather station at the top and down through Méharin to Saint-Esteben. 18km Plus 10 to Saint-Palais. This was a bit sketchy and I’m sure there is a better way.
Day 2. To Espelette. I just took the main roads, which is fine if you keep your wits about you but is tarmac all the way. No worries about navigating though and goes through Hasparren. 25km. There is a lot of traffic on these quite narrow roads.
Day 3. To Irrugne. This is on the Voie Nive Bidassoa and is well baliséed and goes through Ascain. 25km. This is spectacular. Over some substantial hills with plenty to see and varied terrain. The best but toughest day.
Day 4. To Irun. This is on the Voie de la Côte and is well baliséed. An easy day over small hills and through suburbs.
It all worked out all right and was a couple of days shorter than going into SJPdP. But not ideal.