If you look at the early Anglo Saxon kingdoms they were 100 miles across, the Italian city states weren't any bigger. The bronze age kingdoms of Syria were small, similarly the Bronze age kingdoms of Greece. You are used to walking in todays world with paved roads and easy navigation. Sartar is a much less tamed landscape, that requires the traveller to go off road to get to places. The big reason why Alfred the Great drove off the Vikings was because he established across Wessex fortified towns 25 miles or so apart. Saxon armies could march and have a secure source of food every night whereas the Vikings did not. This made the armies of Wessex more mobile than their Vikings opponents. Supporting 5 or 6 people walking 100 miles is relatively easy, supporting an army is much more difficult.