Why do you want to define custom routes for your units? The pathfinding should be finding the fastest route, already, and it will occasionally need to recalculate the route on the fly (if the path is blocked, for example)."
reason for this is because I sometimes your non custom routes, make the people go in strange directions rather than straight to the destination, It is Definitely not calculating the fastest route! Half the time when I'm trying to avoid a camp, the only way it will let me go is through the enemy camp in order to get to my desired destination (reason I made this suggestion). And there is no other way to go to where I want to go other than through the camp according to your games calculations. But if I had a custom route, I could make a straight run for my destination and my troops would be fine. And it would just be fun and strategic.