The calculated distance (air line) between Harrow on the Hill and Southend-on-Sea is approximately 45 miles respectively 73 kilometers.
By car or train, the actual journey to Southend-on-Sea is certainly longer, as only the direct route (as the crow flies) between Harrow on the Hill and Southend-on-Sea has been calculated here.
Air line (approximately)
73 kilometers
39 nautical miles