I have walked from SJPP to Bayonne and can certainly concur that the section along the Nive is flat as a pancake. We more or less used the Voie de la Nive route, which was very pleasant. Undulating but not ridiculously hilly. You could probably stick to roads more if you wanted. At one point we walked through armpit high grass through a field which would be demanding on a bike, but it was only for a hundred metres at most.
When we walked part of the bike path was closed and a detour took us onto a main main road (I wondered if it was a motorway as a couple of people honked at us which made me think we shouldn't have been there. The shoulder was very wide and we walked towards oncoming traffic, but took the first exit we could and muddle around until we found a farmer who redirected us to the river path). I would not be put off by this!