The C&O Canal Towpath runs about 184.5 flat, unpaved miles from Cumberland, MD, to Washington DC. It is described on the Bike Washington website. Most FKT interest has been by cyclists, which makes the route a little unusual for this site.
The supported cycling FKT is 11h41m held by Paul Gruenberger and Ralph Olinger set October 1994, starting in Cumberland. Paul Gruenberger also claimed the unsupported record time of 12h36m set in September 1991, but this was superseded by Chris Shue who rode it in 11h37m on May 19, 2017. Shue's trip report is here: https://chrisshue.com/2017/05/26/surviving-the-canal/ & his Strava track is here: https://www.strava.com/activities/996047551
As far as pedestrians, the legendary Park Barner ran the whole thing (about 300km) in a race in 1976. His time was 36h48m14s, as reported by Nick Marshall in a 1977 article in the Washington Post. The article says this about Barner's run: "Despite facing sub-freezing temperatures last year, Barner ran the length of the canal in 36:48:14, including several hours of sleep after he'd run 122 miles, a period of dehydration when his handlers missed a rendezvous point along the wooded trail and about 50 miles of making his way through a pitch-black night with a flashlight!" More of Barner's remarkable ultrarunning career is detailed here (following his induction into the American Ultrarunning Association Hall of Fame in 2012).
A hiker has mentioned doing the C&O in 144h13m self-supported.