This paved path is the centerpiece of running in downtown Denver. The full length is up to 42 miles. It generally follows a north-south direction, paralleling Speer Blvd. The best section starts at Confluence Park, appropriately named for the confluence with the South Platte River Trail, and heading 4 miles southeast toward the Cherry Creek Mall, going by several parks and landmarks, including the Convention Center, University of Colorado, 16 St. Pedestrian Mall, and three of Denver’s notable performing arts spaces. The two-mile section between Speer/Colfax and Lower Downtown (LoDo) has separate trails for pedestrians and cyclists on each side of the creek. At Speer & 14th, just south of Rt. 40, it’s a nice 1 mile side trip to the State Capitol. Past Cherry Creek Shopping Center, it’s another 1.5 miles one way through a nice linear park to the intersection with S. Colorado Blvd.
For an even longer run, continue another ~7 miles south to Cherry Creek State Park in suburban Englewood, where you’ll find a paved 9-mile reservoir loop and 10 miles of bridle trails through cattail bogs, open meadows and woods, and past a beaver pond. This link has full information for the 42 miles.