Distance: 13.4 miles
Mesa Trail, a 6.7-miler (one way), serves as the main artery to most trails in the Flatirons and forested foothills near Boulder, Colorado. The semi-technical single-track trail runs north to south between Chautauqua Park and Eldorado Springs, and hovers around 6,200-6,300 feet above sea level. Though it doesn’t gain much elevation overall, it dips up and down frequently—prime for interval work. And at overlooks you can catch striking views of the eastern plains. If you’re feeling frisky, take an offshoot trail up Green Mountain or Bear Peak for a lung-busting workout. This adds distance and if you summit, you’ll break the 8,000-foot elevation barrier.