The Rim-to-Rim (R2R), R2R2R, and R2R2R2R2R (! aka 2xR2R2R) have been huge classics for many years. A speed record here really counts as a world-class performance, as many of the best mountain runners have tested their mettle in the Canyon. The fastest times are typically set by running the North and South Kaibab Trails.

Nick Clark posted:

Nick Clark posted the route:

The trailhead for Round is 7 or 8 miles (maybe more) west of Loveland on Hwy 34. Two or 3 miles west of the Big Thompson Narrows, there is a pull-off to the left. This leads to the parking lot for Round. Route goes from trail map to big cairn at Summit: 4.8 miles & 3,000'. 95% singletrack.

Note:  The Round Mountain Trail ascends Sheep Mountain (confusing!)