The calculated distance (air line) between Ganghwa-gun and Seongnam-si is approximately 42 miles respectively 67 kilometers.
By car or train, the actual journey to Seongnam-si is certainly longer, as only the direct route (as the crow flies) between Ganghwa-gun and Seongnam-si has been calculated here.
Air line (approximately)
67 kilometers
36 nautical miles