Caesars legion on the move would need to feed itself from the land a large army needs food and water, basically were looking at an old style army with troops and baggage train, plus these guys are not well armed or armoured they have lots of troops again a failing for them because all you need to do is to deny them food and water, small groups of bos doing hit and run raids on a legion would take huge toll over time, plus they would have casualties along the way trying to force march troops across open country, this isnt europe durig the time of the roman conquests, its a land decimated after a nuclear war, and honestly if they had that sherman tank mobile, it would steamroll the whole army.