The calculated distance (air line) between Katwijk aan den Rijn and The Hague is approximately 10 miles respectively 16 kilometers.
By car or train, the actual journey to The Hague is certainly longer, as only the direct route (as the crow flies) between Katwijk aan den Rijn and The Hague has been calculated here.
Air line (approximately)
16 kilometers
8 nautical miles