The calculated distance (air line) between Saint-Sébastien-sur-Loire and Rennes is approximately 63 miles respectively 101 kilometers.
By car or train, the actual journey to Rennes is certainly longer, as only the direct route (as the crow flies) between Saint-Sébastien-sur-Loire and Rennes has been calculated here.
Air line (approximately)
101 kilometers
55 nautical miles