Then throw in occasional surges of higher intensity levels a couple of days per week. As your fitness level increases, you can increase the time you spend in the higher intensity. Your lower intensity will gradually increase because your heart and muscles will be stronger. This will push your cardiovascular fitness to higher levels. The anaerobic threshold increases, building cardiovascular strength.
Easy workouts allow for rest and endurance-building.
Workouts are varied and interesting. Rest between surges allows you to work out for longer. Injury is avoided. Results are fast. Read more about your heart rate here.