You might want to skip Antietam on your thru-hike because it would be hard to get there and enjoy it without a car. Later you could come back in a car to see Antietam, Gettysburg, and Manassas. All of them are absolutely wonderful.
The battlefields keep getting better. At Antietam, they put in a trail so you can walk across the Cornfield. At Gettysburg, they buried the power lines that ran along the Emmittsburg Road and cut down many of the trees between that road and Cemetery Ridge. Manassas is a huge battlefield with miles and miles of hiking trails.
Don’t come in the summer. The heat is too oppressive. The Fall or Spring are much nicer.
But, if you insist on going to Antietam, be sure to see the short movie they show. I must have watched it 15 times.
Bob J