The calculated distance (air line) between Frýdlant and Mladá Boleslav is approximately 36 miles respectively 58 kilometers.
By car or train, the actual journey to Mladá Boleslav is certainly longer, as only the direct route (as the crow flies) between Frýdlant and Mladá Boleslav has been calculated here.
Air line (approximately)
58 kilometers
31 nautical miles