Hi Iago,
I walked the route in 2014. I had limited time, so I powered through it in two days. I slept alone in Helette at the end of day one in the
gite municipal, which was basic but had a good kitchen. Then I continued on to SJPdP the next day. I used
this site for maps and instructions.
For the most part, the routing was straightforward. Some bridge repair made a mess of things outside of Bayonne, but I suspect that will be resolved by the time you walk. I lost the route around 10km before St. Jean, but that may have been my fault. It was no problem, though--I just followed a local road for a few km and got back on track.
On the whole, it's not the most exciting walk, and there's a lot of time on asphalt. But, it has its moments. Note that if you're interested in walking from the airport, you can follow the Voie Littorale from the Biarritz train station (very close to the airport) back to Bayonne, and then carry on from there on the Nive (the two routes split outside of Bayonne).
Walking from Bayonne to Hendaye and then back to SJPdP would take a full week. It's more spectacular, as you can follow the coast south and then the Pyrenees inland via the GR-10 (or a lower level trajectory north of it), but the GR-10 in particular is much more demanding.
If you're just interested in getting in a few days of walking before the Pyrenees, I might suggest traveling from Bayonne to Navarrenx and then following the Le Puy route from there. That's three easy days of walking with some gradually increasing foothills.
More info than you wanted, I'm sure!
Dave