The calculated distance (air line) between Haarlem and Alphen aan den Rijn is approximately 17 miles respectively 28 kilometers.
By car or train, the actual journey to Alphen aan den Rijn is certainly longer, as only the direct route (as the crow flies) between Haarlem and Alphen aan den Rijn has been calculated here.
Air line (approximately)
28 kilometers
15 nautical miles