The calculated distance (air line) between Tuen Mun San Hui and Mong Kok is approximately 14 miles respectively 22 kilometers.
By car or train, the actual journey to Mong Kok is certainly longer, as only the direct route (as the crow flies) between Tuen Mun San Hui and Mong Kok has been calculated here.
Air line (approximately)
22 kilometers
12 nautical miles