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