The calculated distance (air line) between Mendig and Hennef (Sieg) is approximately 28 miles respectively 45 kilometers.
By car or train, the actual journey to Hennef (Sieg) is certainly longer, as only the direct route (as the crow flies) between Mendig and Hennef (Sieg) has been calculated here.
Air line (approximately)
45 kilometers
25 nautical miles