While Jack was going through the procedurally generated maze dungeon under the royal mausoleum, John was leading the army through the mountainous terrains of the Jagara region.
When he was back in Thereath mapping the route for the army to take, he had mapped a second route for backup. He was now using this backup route.
The first route was a wide path with the least concentration of monsters. Hence, most natives who had ventured out into the wilderness should know this route as well. It was the most common route to take if one wanted to go through the Jagara region quickly. This also made the path the easiest to intercept if someone wanted to stop their advance.
He expected his mother to have prepared some kind of an obstacle for him on this path. He just didn't expect it to be a Sangrod army.