The calculated distance (air line) between Donostia / San Sebastián and Tarnos is approximately 30 miles respectively 49 kilometers.
By car or train, the actual journey to Tarnos is certainly longer, as only the direct route (as the crow flies) between Donostia / San Sebastián and Tarnos has been calculated here.
Air line (approximately)
49 kilometers
26 nautical miles