The calculated distance (air line) between Bayt Sāḩūr and Idhnā is approximately 18 miles respectively 29 kilometers.
By car or train, the actual journey to Idhnā is certainly longer, as only the direct route (as the crow flies) between Bayt Sāḩūr and Idhnā has been calculated here.
Air line (approximately)
29 kilometers
15 nautical miles