The calculated distance (air line) between Kostanjevica na Krki and Maribor is approximately 50 miles respectively 81 kilometers.
By car or train, the actual journey to Maribor is certainly longer, as only the direct route (as the crow flies) between Kostanjevica na Krki and Maribor has been calculated here.
Air line (approximately)
81 kilometers
44 nautical miles