I live in Northern Virginia, close to the border of Maryland. The Potomac River separates Virginia and Maryland in this part of the state. Where I live is very close to one of the few bridges that cross the Potomac, so it's only about a 10 minute drive for me to be on the Maryland side of the Potomac River. Why does this matter? Well, that's where the C&O Canal Towpath is!
C&O (Chesapeake & Ohio) Canal Towpath - Brief History (NOT Fact Checked)
Way back in "the day", our forefathers wanted to use the Potomac River to get goods west from DC. The Potomac River is obviously a good option, but, it isn't the most easily navigable river (rapids, and whatnot), and it flows TOWARD DC, so they built a canal next to the river (think Panama Canal idea, but way WAY smaller scale) to allow boats loaded with cargo to west. They got as far as Cumberland, MD where the Great Allegheny Passage (GAP) starts. At this point trains took over and made the canal obsolete. Luckily the C&O Canal was made a National Park in the 1930's, and the towpath, which is a small dirt/gravel road used to pull the boats along the canal (think donkeys with ropes) was restored and kept in good condition.
I stumbled across the towpath one day while in Point of Rocks and thought it would be a good place to run on a softer surface. Recently, I got the bright idea to run the full length of the towpath (185 miles from DC to Cumberland MD), in small chunks. Something to make my long runs interesting and see first hand some US history. By the time I'm done I will have run twice the length of the path (that would be 370 miles) because each run has to be "out and back" since I'll be driving myself to various starting points along the path.
How Far Have I Gotten So Far? (Jan. 1, 2016)
So far I've run the path from mile marker 33 (by White's Ferry - an actual rope ferry from the 18th century that's still in operation) to mile marker 69 (which is almost to the Antietam Civil War Battlefield). My first runs started at Point of Rocks (mile marker 48) and I've been "radiating out" in either direction from that point.
I've passed a lot of pretty cool structures and buildings, including aqueducts, "lock houses", docks for the river, Civil War history, the Appalachian trail, and trains (of course when trains became a thing the canal was a perfect place for tracks - a level path was already in place).
So...I have a LONG way to go to cover the full length of the path, but I'll get there one day.