The airship has a maximum speed of 69 mph. By traveling at 60 mph, I was able to make the following observations:
The Gettysburg map is approximately 4.5 miles x 4.5 miles
Infantry travels at approximate 12.5 mph
Horses travel between 25-30 mph (I couldn't get a more precise reading because I accidentally rode off the edge of the map when I was about 85% through the trip, and I wasn't about to spend the 20 minutes of riding through empty fields to double-check my measurements).
Here is what the edge of the map looks like:
Going off the edge will kill you.
The above fact is the easiest way to destroy enemy ships.
Only five more hours to go. I think I'm going to make it.