The calculated distance (air line) between Delft and Alphen aan den Rijn is approximately 15 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 Delft and Alphen aan den Rijn has been calculated here.
Air line (approximately)
25 kilometers
13 nautical miles