The calculated distance (air line) between Sint-Martens-Lennik and Leuven is approximately 24 miles respectively 38 kilometers.
By car or train, the actual journey to Leuven is certainly longer, as only the direct route (as the crow flies) between Sint-Martens-Lennik and Leuven has been calculated here.
Air line (approximately)
38 kilometers
21 nautical miles