Use pace as your primary intensity metric when running at high intensity (Zones 4 and 5), except when running uphill, in which case you should rely on perceived effort. Use perceived effort to fine-tune your intensity during your most challenging workouts. Specifically, use it to ensure you finish all such runs feeling you could have run at least a little faster or farther. Use perceived effort to adjust your intensity on “flat” days. Specifically, when you feel worse than normal at a given target heart rate or pace, slow down.
