Kris Stallenberg 90-min Z2 (Endurance) Medium #03 This session uses 'Over-Unders' entirely within your endurance zone to simulate rolling terrain. By alternating between the lower and upper ends of Zone 2, you'll improve your ability to handle subtle changes in pace without leaving the aerobic state. This is a great way to break up a long endurance ride and keep it engaging. According to the Coggan Power Zones, this variability trains metabolic flexibility. bike