Actually, as you probably know, beaches in the Landes are very long. In fact, the Landes coastline is one 106-km long beach!
So, once you have joined a quiet beach* by its main access, you can walk in one or other direction along the tideline to get away from everyone else. Make sure you use the beach access ramps provided to avoid walking across the dunes.
*Away from supervised bathing areas, naturism is tolerated on the following beaches: