The calculated distance (air line) between Katwijk aan den Rijn and Dordrecht is approximately 29 miles respectively 46 kilometers.
By car or train, the actual journey to Dordrecht is certainly longer, as only the direct route (as the crow flies) between Katwijk aan den Rijn and Dordrecht has been calculated here.
Air line (approximately)
46 kilometers
25 nautical miles