This is from the Denver Water website:
The High Line Canal, designated as a National Landmark Trail, was completed in 1883 to deliver irrigation water. Today, the canal is owned and operated by Denver Water. The canal trail has become a major recreational amenity in the Denver area, as people enjoy using the canal’s trail for hiking, biking, jogging and horseback riding.
The Guide to the High Line Canal Trail is available in hard copy or pdf format.
The canal itself runs 71 miles from a diversion dam in Waterton Canyon south of Denver, to the Rocky Mountain Arsenal Lateral in the Green Valley Ranch area in northeast Denver. However, the first 1.7 miles (in Waterton Canyon) and the last 5 miles are not accessible to trail users, so the High Line Canal Trail (HLCT) is 64.3 miles long. As such, it is surely one of the longest suburban recreation trails in the United States. It provides a scenic and extremely indirect tour of Denver's southern, eastern and northeastern suburbs.
From Mile 1.7, where the trail starts at Waterton Road, to about Mile 37, the trail is almost all dirt. From Mile 37 to about Mile 50 the trail is mostly asphault, and from Mile 50 to Mile 66 the surface is primarily cement. However, it is almost always possible to run on dirt along side of the paved sections of the trail.
With one exception, the route is obvious -- you just follow the canal. Any questions are quickly cleared up by consulting the Denver Water guide. The tricky bit comes at Mile 8.6 where the canal disappears into a siphon to cross an extensive low-lying wetland area. The work-arounds for this section are long, cumbersome and unaesthetic (private property, busy roads). The obvious choice is to simply cross the wetland, which is of course both really hard and un-ecological -- unless the wetland happens to be frozen, such as on a cold January morning, when the crossing is direct and straightforward. After the wetland and some private property, the trail resumes at Mile 9.8.
Resupply stops (mini-marts) are passably frequent along the HLCT.
This is a surprisingly good route! Up to about Mile 50 it is especially nice -- rural, peaceful, lined with cottonwoods, and with frequent big views of the city and the Front Range of the Rocky Mountains to the west. After 50 it becomes a little more "urban" feeling, though part of that is probably the cement. Besides being tough on the legs, running on cement just feels more urban because you get the impression you are on a sidewalk. Around Denver, everyone has run or biked on the HLCT, but it is rare to hear of anyone running the whole thing. While 64 miles is definitely long, the trail is basically flat, the surface is good, and resupply is easy, so this is a good winter project -- especially if you can get someone to drop you off and pick you up!
The people who built the High Line Canal more than a century ago didn't envision that people would be using their ambitious project as a recreational outlet in the midst of a busy urban area. In fact, to the builders of the 71-mile High Line, the canal was solely a commercial scheme to bring South Platte River water to settlers for drinking and farmers for agricultural purposes following a gold rush in 1859 near the confluence of the South Platte and Cherry Creek. -- From the Denver Water "Guide to the High Line Canal Trail"
Making an FKT attempt on this route self supported this coming Friday/Saturday June 12-13 2020.
Update: Raccoon ate all of my drop bag fuel and ruined my water at mile 22, decided to pull the plug at 50k as I didn't have supplies to make it to my next drop at mile 40 and no convenience stores that I ran by were open around 12-1 am. Glad I did as I went to retrieve drop bag 2 around mile 40 this AM and also had been destroyed by raccoons. Maybe attempt in cooler weather during normal business hours later this year.