The Burro Trail runs for about 4 miles one-way, although it has a few branches, runs through a network of unmarked trails near Breckenridge, and connects to other trails on its upper end.
You can either ride this trail from town as an out-and-back, or loop it with the highway and the dirt Spruce Creek Road. I personally incorporated it into a much longer ride with several other trails. The trailhead given is for the upper end of the trail off of Spruce Creek Road.
The upper portion of the trail is located on an old logging road grade that has since narrowed down into mostly singletrack. It's a fast but pedally descent with lots of loose rocks.
Once it takes a right turn off the road bed, things get interesting with some fast, swoopy singletrack descending through a vast network of mostly unmarked singletrack. Choose your own adventure, keep going downhill, and eventually you will end up in Breckenridge!