At Mile High Run Club, there's no shame in runners taking a break. In fact, breaks are built into the treadmill-based workouts at the fitness studio, which specializes in interval training. In their signature "Dash 28" class, for instance, participants run up to three miles on an interval basis, then finish with 10 minutes of strength training and kettlebells. In their "The Distance" class, meanwhile, participants run intervals for an hour, led by elite runners. The full array of classes can accommodate novice runners as well as more experienced athletes training for races and marathons.