Summit County is a popular recreation area west of Denver, known for skiing in winter and hiking in summer. Resorts in the area include Breckenridge, Copper, Keystone, and Arapahoe Basin. 75% of Summit County is public land.
Here’s the situation for running in Summit County: I-70 is the main road, with mountains all around. Naturally, the trail running options are extensive. For easier road running in the valleys, there’s a 60-mile network of multi-use paths, with good options in Silverthorne, Dillon, Frisco, and Breckenridge. Among the highlights are the paths around the Dillon Reservoir, the Blue River Bikeway, and the Tenmile Canyon Recreational Trail between Breckenridge and Vail.
Note that this entire area is at altitude. All of the towns are at least 7,000 feet. Note also that the paths might be snow covered (and used for XC skiing) in winter. Paths are generally clear May-October, and are especially beautiful in fall with the changing aspens.