The calculated distance (air line) between Gyeongsan-si and Naesŏ is approximately 41 miles respectively 67 kilometers.
By car or train, the actual journey to Naesŏ is certainly longer, as only the direct route (as the crow flies) between Gyeongsan-si and Naesŏ has been calculated here.
Air line (approximately)
67 kilometers
36 nautical miles