The calculated distance (air line) between Pyeongtaek and Naesŏ is approximately 145 miles respectively 233 kilometers.
By car or train, the actual journey to Naesŏ is certainly longer, as only the direct route (as the crow flies) between Pyeongtaek and Naesŏ has been calculated here.
Air line (approximately)
233 kilometers
126 nautical miles