Day 1: We went to Frostburg to hike the rail trail there. Our conclusion was that it makes a good bike trail but not a good hiking trail because it is fairly flat and boring for walking.
Day 2: On the advice of the hotel person, we spent a lot of time looking for Dan's Mountain State Park, which does not actually exist (it is a recreation area with one hiking trail designed for kids and a nearby scenic view called Dan's Lookout Rock or some such). So we backtracked down 68 and 70 and spent the remainder of the day wandering Antietam. Have never been, and I am glad we went.
My father observed that Antietam was just enough of a victory for Lincoln to announce the forthcoming Emancipation Proclamation (which would issue 3 months later) and shift U.S. policy more explicitly to the abolition of slavery (as opposed to crushing the rebellion and leaving all other "institutions" intact) and keep France and England out of the war, but not so much of a victory that Lincoln would continue to pursue the war without the Emancipation Proclamation.
All in all, it was a very nice time. Next time I will plan the hiking better.