The calculated distance (air line) between Katwijk aan den Rijn and Amersfoort is approximately 41 miles respectively 66 kilometers.
By car or train, the actual journey to Amersfoort is certainly longer, as only the direct route (as the crow flies) between Katwijk aan den Rijn and Amersfoort has been calculated here.
Air line (approximately)
66 kilometers
36 nautical miles