Near Breckenridge, the 20 mile Flume Trail is an excellent mountain bike route. It does cross some private property.
To reach the mountain biking trailhead, followUS 70 to SR 9 in Frisco. Head south 5.5 miles to east on Tiger Road. After 5.7 miles, stay right at junctions and follow sign to Middle/South fork of the Swan Jeep Road. Park after 0.5 miles at the fork.
Overall, Flume Trail is the 26th most popular mountain bike trail of all 175 mountain biking rides in Colorado.