When we talk about workouts at a specific pace (however you define them...vo2, threshold, marathon) there are key workouts that crop up in many different plans and approaches: 20x400, 5x1k, 20 miles at MP). My question is what is the general range for time spent at a certain pace to maximize improvement of that energy system? This is similar to in weightlifting many use sets of 12 reps, but in running I’ve never really read about or understood what makes the volume workout super important to enhance race performance.