Les Landes sont traversées par les 4 chemins de St Jacques de Compostelle : voie de Tours, voie du Puy, voie de Vézelay et voie du Littoral.
www.compostelle-landes.org
I think their initial marked route goes on the inland side of the lake via Sanguinet instead of your plan to keep to the coast to Biscarrosse Plage.
Two comments - it early in the season for the "resort" towns along the coast so not everything will yet be open. Its also Easter weekend so there will also be closures of shops, restaurants etc. So make sure you have plenty of food with you.
And just in case you are interested I wrote up about my journey
End-August through mid-September, I walked between Toulouse and Santander. From Toulouse I walked along the very nice Via Garonne passing several beautiful French villages - not least ending in...
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