Denver’s colfax marathon goes big on fun (a jaunt through Sports Authority Field at Mile High) and support (13 aid stations). No wonder more than 16,000 runners jammed the starting line this year. But when it comes to adventure…meh. “They’re handing you sports drinks, and if you fall and twist an ankle, there’s an ambulance nearby,” says Boulder ultrarunner Buzz Burrell. “With FKTs, if something goes wrong, it’s all on you.” An FKT is a “fastest known time,” an unofficial race against the clock, often through a remote area. Burrell and fellow Boulderite Peter Bakwin picked up the term in 2000 when, before tackling California’s 223-mile John Muir Trail, they wondered who had run it fastest. After tracking down the record via word of mouth (and crushing it), Bakwin launched, a clearinghouse where runners track FKTs. They choose routes, look up the records, post their intentions to topple them, and use GPS trackers, selfies, and/or video to validate their times. While you might not be ready for Colorado’s most arduous FKT (that’d be a 100-mile route over 14 different fourteeners in 60 hours called Nolan’s 14), consider a few of the Centennial State’s more popular routes.

Kokopelli Trail

Where: From Loma to Moab, Utah
Distance: 141 miles
FKT: 30 hours, 20 minutes, 58 seconds, by 45-year-old Highlands Ranch pharmaceutical rep Scott Jaime and Boulder space weather scientist Nick Pedatella
Highlight: A scenic desert landscape and, once in Utah, Manti-La Sal National Forest

Colorado Trail

Where: From Durango to Waterton Canyon, southwest of Denver
Distance: 486 miles, gaining 89,354 feet in elevation
FKT: 8 days, 7 hours, 40 minutes, by Jaime
Highlight: This route’s 28 access points make it easy for saner people to run or hike only a portion of it.

Four Passes Loop

Where: Circumnavigates the Maroon Bells near Aspen, starting and finishing at the Maroon Lake trailhead
Distance: 28-mile loop
FKT: For women: 6 hours, 28 minutes, 59 seconds, by Boulder artist Sandi Nypaver; for men: 4 hours, 27 minutes, 10 seconds, by Boulder running coach Sage Canaday
Highlight: Four 12,000-foot passes, panoramic views of the sheer Elk Mountains (including the Bells and imposing fourteener Pyramid Peak), and beautiful Crater and Snowmass lakes

Towers Road

Where: Soderberg Trailhead inside Horsetooth Mountain Open Space, west of Fort Collins
Distance: 3.4 miles, gaining 1,700 feet
FKT: 28 minutes, 27 seconds, by Fort Collins resident James Vance
Highlight: Relatively short but hardly sweet, once Towers starts rising it doesn’t quit until you reach the top, which explains why it’s a favorite training route for runners and power-hikers.

Mt. Sanitas

Where: Just west of Fourth Street on Mapleton Avenue in Boulder
Distance: 1.4 miles one way, gaining 1,300 feet
FKT: 14 minutes, 12 seconds, by Spanish long-distance runner Kilian Jornet
Highlight: This classic test of will rewards you with stellar views of Boulder, the Flatirons, and the towering Indian Peaks from the summit—if you can get the sweat out of your eyes.

(Read what three Colorado record-holders love about their FKT routes)