I know of route from Pamplona to Irun on the Camino Norte using the Camino Baztan for part of it. I have not walked it, just plotted it out.
From Pamplona walk the Camino Baztan in reverse for two Gronze stages to Berroeta. From there you go off camino following the highway N-121a north for about 7 kilometers to Oiregi where the Via Verde del Bidasoa starts on its 44 km way to Irun, about a marathon distance away.
Via Verdes are old railways converted to walking and cycling routes, what we in the US call rail trails. So it should be easy walking. And it is downhill to Irun. Also, if you use Google Maps to plot a walk from Berroeta to Oiregi you see it plots a way following the highway but not on it. It uses smaller parallel roads.
I think there is lodging along the way but most likely off the trail.
For more information on the Via Verde del Bidasoa visit this webpage:
Vías Verdes
www.viasverdes.com
Bidasoa Greenway - Vías Verdes de España
www.viasverdes.com