The calculated distance (air line) between Tonyrefail and Merthyr Tydfil is approximately 12 miles respectively 19 kilometers.
By car or train, the actual journey to Merthyr Tydfil is certainly longer, as only the direct route (as the crow flies) between Tonyrefail and Merthyr Tydfil has been calculated here.
Air line (approximately)
19 kilometers
10 nautical miles