The calculated distance (air line) between Hernani and Saint-Jean-de-Luz is approximately 18 miles respectively 29 kilometers.
By car or train, the actual journey to Saint-Jean-de-Luz is certainly longer, as only the direct route (as the crow flies) between Hernani and Saint-Jean-de-Luz has been calculated here.
Air line (approximately)
29 kilometers
16 nautical miles