Goal: increase aerobic work capacity without accumulating fatigue
How: stay in the middle of the blue zone. The duration will depend on your goal: a few minutes for a warm up, 15-20 minutes for a recovery session, or 45-60+ minutes to develop your aerobic system.
Exercises: you can use any cardiovascular exercise—from sled pushes to rowing, to biking—so long as you keep your heart rate in the middle of the blue zone.