The calculated distance (air line) between Longueuil and Saint-Hyacinthe is approximately 26 miles respectively 42 kilometers.
By car or train, the actual journey to Saint-Hyacinthe is certainly longer, as only the direct route (as the crow flies) between Longueuil and Saint-Hyacinthe has been calculated here.
Air line (approximately)
42 kilometers
23 nautical miles