The calculated distance (air line) between Saint-Josse-ten-Noode and Mons is approximately 33 miles respectively 53 kilometers.
By car or train, the actual journey to Mons is certainly longer, as only the direct route (as the crow flies) between Saint-Josse-ten-Noode and Mons has been calculated here.
Air line (approximately)
53 kilometers
29 nautical miles