The calculated distance (air line) between Pyŏksŏng-ŭp and Uiju is approximately 159 miles respectively 255 kilometers.
By car or train, the actual journey to Uiju is certainly longer, as only the direct route (as the crow flies) between Pyŏksŏng-ŭp and Uiju has been calculated here.
Air line (approximately)
255 kilometers
138 nautical miles