The calculated distance (air line) between Sliven and Veliko Tŭrnovo is approximately 45 miles respectively 72 kilometers.
By car or train, the actual journey to Veliko Tŭrnovo is certainly longer, as only the direct route (as the crow flies) between Sliven and Veliko Tŭrnovo has been calculated here.
Air line (approximately)
72 kilometers
39 nautical miles