The calculated distance (air line) between That Phanom and Sakon Nakhon is approximately 40 miles respectively 65 kilometers.
By car or train, the actual journey to Sakon Nakhon is certainly longer, as only the direct route (as the crow flies) between That Phanom and Sakon Nakhon has been calculated here.
Air line (approximately)
65 kilometers
35 nautical miles