The calculated distance (air line) between Riihimäki and Lappeenranta is approximately 117 miles respectively 188 kilometers.
By car or train, the actual journey to Lappeenranta is certainly longer, as only the direct route (as the crow flies) between Riihimäki and Lappeenranta has been calculated here.
Air line (approximately)
188 kilometers
101 nautical miles