The calculated distance (air line) between The Hague and Alphen aan den Rijn is approximately 16 miles respectively 25 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 The Hague and Alphen aan den Rijn has been calculated here.
Air line (approximately)
25 kilometers
14 nautical miles