The calculated distance (air line) between Darmstadt and Marburg an der Lahn is approximately 65 miles respectively 105 kilometers.
By car or train, the actual journey to Marburg an der Lahn is certainly longer, as only the direct route (as the crow flies) between Darmstadt and Marburg an der Lahn has been calculated here.
Air line (approximately)
105 kilometers
56 nautical miles