Long distance communication in the ancient world
was better than one might think: they really did use the "Lord of the Rings" method of having signal towers at regular intervals. For instance, the Romans had them along the "Saxon Shore," which was a defense system along the shore of east and southeast England. So you could signal that, say, "The Picts are coming" from Hadrian's Wall to Kent at the speed of light, plus the time it would take for the human at each signal station to relay the signal.