The armies of the north were clearly not expecting the Israelites to come and attack them. They were confident in their numbers and military might and the camp of Israel was far south. In order to come upon them suddenly, Israel must have journeyed during the night so that they would remain hidden. This is what they did in Joshua 10:9 in order to attack the southern army. From the camp at Gilgal (Joshua 10:43), the journey north to the waters of Merom was at least sixty miles (one hundred kilometres). The armies of Israel would not be able to travel this distance in one night. It would have taken at least three nights of travel, possibly more if they could not make use of the main roads (in order to avoid detection).
7 So Joshua and all his warriors came suddenly against them by the waters of Merom and fell upon them.