The outer loop of the Back 40 Trails system in Bella Vista, Arkansas. 21 miles, 99% singletrack. The loop starts and ends at mile marker 0 for the Back 40 loop (pictured below) at the Blowing Springs park entrance. 

The Zurich to Zug route connects two major cities in Switzerland and traverses the Albis Mountain Chain, climbing important local peaks such as the Uetliberg and Albishorn. The route covers a total distance of 34km with just over 1,000m of height gain.

Christopher Thomas submitted the route, which covers all of the Rim to Reno Trail with the summit of Mt Rose as an out & back.  About 40 miles:

https://parks.des.qld.gov.au/parks/carnarvon-great-walk/

86k loop of remote technical single track in central Queensland, Australia. 

https://www.summitpost.org/celo-knob/154302

"Celo (pronounced SEE-low) Knob is one of the most remote of our Southern sixers. No matter how you get to the summit, you must either make a climb in excess of 3,000 feet, and/or a long and arduous trek along miles of the most rugged trails in the East."

This is a Premier Route. 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.

Via Alpina -  Original Red Trail is a 2650K trail that goes through all Alpines countries.
The trace is fully specified and detailed on the official website of the trail: