The calculated distance (air line) between Marseille and Aix-en-Provence is approximately 16 miles respectively 26 kilometers.
By car or train, the actual journey to Aix-en-Provence is certainly longer, as only the direct route (as the crow flies) between Marseille and Aix-en-Provence has been calculated here.
Air line (approximately)
26 kilometers
14 nautical miles