The calculated distance (air line) between Mechelen-aan-de-Maas and Mons is approximately 84 miles respectively 135 kilometers.
By car or train, the actual journey to Mons is certainly longer, as only the direct route (as the crow flies) between Mechelen-aan-de-Maas and Mons has been calculated here.
Air line (approximately)
135 kilometers
73 nautical miles