The aim of this session is to develop your threshold running. This session is known as a under-over session, meaning we're doing 4 x 6 minute reps as 2 minutes below, 2 minutes above, and 2 minutes below threshold place. This variable pace stresses your physiology and pushes up your threshold speed. Threshold is the point at which the powerhouse of the muscle cell (mitochondria) can no longer produce energy (ATP) aerobically. As such, threshold training is performed at a high intensity, somewhere around 7 or 8 out of 10 in terms of perceived intensity. Be sure to have your legs tip-top before getting stuck into this session! PS. It's also a great session to do off the bike.