Coming from MI ?  I would not bother; unless; you are very interested in Civil War history; or are visiting somewhere close to Gettysburg.

There is maybe 2 days riding there at walking speed.

I have ridden there lots and lots of times, but lived just south of there and it was not very far from home.

As a civil war cavalry reenactor, I enjoy Gettysburg immensely, and never get tired of riding the battlefield. I am also a member of the Gettysburg Equestrian Historical Society, responsible for trail maintenance. The loop is 3 hours at a walk on a quarter horse. I have ridden the loop 9 times so far this year. An idea would be to stay at the Artillery Ridge Campground, ride Gettysburg, then trailer to Antietam a day, then Manassas another day. Antietam is mostly riding along the sides of the park roads. You do, however, get to ride over Burnside Bridge, a big deal for us history buffs. With Manassas, The Equestrian Society of Manassas does a phenomenal job of trail maintenance. Lots of MULCH of some trails. You can easily ride 6 hours doing all the loops.....even more if you start at Bull Run State Park and ride thru the tunnel under the interstate to the Manassas trails. You could then trailer from Manassas to Fort Valley, Virginia Horse Camp (no more than 90 minutes, probably less) to enjoy days of mountain trail riding.

Hope this helps!

Brenda