Paria Canyon and its tributary, Buckskin Gulch, are two popular and amazing routes the Paria Canyon - Vermillion Cliffs Wilderness. The 38-mile Paria Canyon route is normally done as a 3-5 day backpacking trip. It begins at the Whitehouse Trailhead off US Highway 89 between Kanab, UT, and Page, AZ, and ends at Lee's Ferry, AZ. Buckskin Gulch has been called the longest slot canyon in the world -- it is 15+ miles of pure slot!
Mee & Knowles are two spectacular and remote canyons in the Black Ridge Canyons Wilderness of the McInnis Canyons National Conservation Area in western Colorado. If you like rugged desert landscape and don't want to see other people, this route is for you! Closing the loop requires around 38 miles of on- and off-trail hiking/running with somewhat tricky navigation. The route is described in Michael Kelsey's Canyon Hiking Guide to the Colorado Plateau.
The Maroon Bells Four Passes Loop is a 28-mile route in the Elk Mountains near Aspen, Colorado. It circumnavigates the Maroon Bells, through some of the most spectacular scenery in the state. I think it just might be the best run in the entire state! It is a challenging route, with 7,500 feet of climbing and descending, numerous creek crossings, and altitudes up to 12,460 feet.
The Pawnee Pass (12,541') - Buchanan Pass (11,837') Loop is a terrific, tough and super scenic 26-mile route in Colorado's Indian Peaks Wilderness. Here is a nice trip report on the route from Charles Danforth (from which I took the map and photo). More photos are here.
Matt Hart established a new route circumnavigating the Teton Range of Wyoming, that looks to be an instant classic. It is 33.7 miles with 8,165 feet of gain (according to the topo).
The Uinta Highline Trail runs the length of the high Uinta mountain range through the High Uintas Wilderness in northeastern Utah. It is a true wilderness trail, and does not intersect a road for at least 70 miles. The trail is around 100 miles long, but most backpacking trips are done over a 70 mile section between Chepeta Lake and Hayden Pass.
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.
This is a Premier Route (Standard Route, one way). To claim an FKT on this route, you must meet a high standard of documentation and verification. See details here: https://fastestknowntime.com/premier-routes.
If I can just get off of this LA freeway
Without getting killed or caught
I'd be down that road in a cloud of smoke
For some land that I ain't bought bought bought
- Guy Clark (L.A. Freeway)
“In the late 1950s Carl Pfiffner spoke passionately about traversing from the Arapaho Peaks to Longs Peak along the Continental Divide. This project retains all of its original mystique and it the ultimate mountaineering adventure in the Front Range. The adventure can be extended by going from Berthoud Pass to Milner Pass. There are many variations to this complicated project and any attempt requires your creativity.” - Gerry Roach, “Colorado’s Indian Peaks Wilderness Area, Classic Hikes & Climbs” (1989)