The calculated distance (air line) between Ongjin and Kangdong-ŭp is approximately 92 miles respectively 149 kilometers.
By car or train, the actual journey to Kangdong-ŭp is certainly longer, as only the direct route (as the crow flies) between Ongjin and Kangdong-ŭp has been calculated here.
Air line (approximately)
149 kilometers
80 nautical miles