When were you intending for the enemy army to arrive? How far away is the enemy city? Does the enemy army have flying scouts of its own?

If Dr. Doom flies west for 48 straight hours, he'll cover over 300 miles. The enemy army, by contrast is at least a couple of weeks away. Do any of the players care if you just let 4 days pass uneventfully? Is it possible Dr. Doom will run into a squadron of enemy flyers long before he gets two days out?