The calculated distance (air line) between Pyŏksŏng-ŭp and Hŭngnam is approximately 167 miles respectively 268 kilometers.
By car or train, the actual journey to Hŭngnam is certainly longer, as only the direct route (as the crow flies) between Pyŏksŏng-ŭp and Hŭngnam has been calculated here.
Air line (approximately)
268 kilometers
145 nautical miles