The calculated distance (air line) between Mäntyharju and Lappeenranta is approximately 50 miles respectively 80 kilometers.
By car or train, the actual journey to Lappeenranta is certainly longer, as only the direct route (as the crow flies) between Mäntyharju and Lappeenranta has been calculated here.
Air line (approximately)
80 kilometers
43 nautical miles