The Vélodyssée is a 1,200 km cycle path that connects Roscoff in Brittany with Hendaye in the Basque Country. It comprises 14 sections.
70 % of the route is on traffic-free paths. The Dune du Pilat lies on the 12th section Arcachon - Léon (129.6 km). If you want to see the dune and come by bike, you don't have any expenses (no parking fees like for cars, no entrance fee).
The website about this cycle path (véloroute) contains all the information you need to plan your trip (info in English and French).
For more information:
Cycle paths near the Arcachon bay: