Heart rate zones are tools, not rules. Seattle-based Ironman coach Steven Pust explains why most runners get stuck in the Zone 3 gray zone—where easy runs are too hard and hard workouts aren't hard enough. Learn how to preserve interval integrity, balance intensity with recovery, and use heart rate training to improve performance without burnout. Whether you're training for marathons, triathlons, or just trying to stop feeling tired all the time, discover how polarized training and proper zone usage can transform your results.