The calculated distance (air line) between Katwijk aan den Rijn and Leeuwarden is approximately 91 miles respectively 146 kilometers.
By car or train, the actual journey to Leeuwarden is certainly longer, as only the direct route (as the crow flies) between Katwijk aan den Rijn and Leeuwarden has been calculated here.
Air line (approximately)
146 kilometers
79 nautical miles