Here's the shortest overland route that I could trace that exists at the end of the game:

Igros Castle
Mandalia Plains
Gariland Magic City
Murond Holy Place
Goug Machine City
Zigolis Swamp
Lionel Castle
Bariaus Valley
Warjilis Trade City
Deep Dungeon

It would take nine days to traverse this route.

As for the trick, does it involve the point in the game where a sea route connects Warjilis Trade City and Goug Machine City? If the sea route were an option by the time Deep Dungeon appeared you could eliminate Zigolis Swamp, Lionel Castle, and Bariaus Valley from the route, which would cut the trip to six days.