The calculated distance (air line) between Pietermaritzburg and Piet Retief is approximately 182 miles respectively 293 kilometers.
By car or train, the actual journey to Piet Retief is certainly longer, as only the direct route (as the crow flies) between Pietermaritzburg and Piet Retief has been calculated here.
Air line (approximately)
293 kilometers
158 nautical miles