The calculated distance (air line) between Pyŏksŏng-ŭp and Kaesŏng is approximately 55 miles respectively 88 kilometers.
By car or train, the actual journey to Kaesŏng is certainly longer, as only the direct route (as the crow flies) between Pyŏksŏng-ŭp and Kaesŏng has been calculated here.
Air line (approximately)
88 kilometers
47 nautical miles