Kris Stallenberg 30-min Z2 (Endurance) Medium #02 This session introduces gentle variations within your endurance zone. By alternating between the lower and upper ends of Z2, you'll improve your ability to handle subtle changes in pace and terrain. This 'over/under' style workout, guided by the principles of Coggan Power Zones, helps build aerobic resilience and keeps the mind engaged during endurance training. bike