According to AllTrails.com, the longest walking trail in District of Columbia is C & O Canal Towpath Trail. This trail is estimated to be 48.0 mi long.
District of Columbia: With an ascent of 1,112 ft, Western Ridge Trail has the most elevation gain of all of the walking trails in the area. The next highest ascent for walking trails is Palisades, Glover Archbold, and Woodley Loop with 941 ft of elevation gain.