The calculated distance (air line) between Serhetabat and Qala i Naw is approximately 49 miles respectively 78 kilometers.
By car or train, the actual journey to Qala i Naw is certainly longer, as only the direct route (as the crow flies) between Serhetabat and Qala i Naw has been calculated here.
Air line (approximately)
78 kilometers
42 nautical miles