The calculated distance (air line) between Yuen Long Kau Hui and Mong Kok is approximately 13 miles respectively 20 kilometers.
By car or train, the actual journey to Mong Kok is certainly longer, as only the direct route (as the crow flies) between Yuen Long Kau Hui and Mong Kok has been calculated here.
Air line (approximately)
20 kilometers
11 nautical miles