Old Way: Intervals at 5K race pace
New Way: Know the purpose of the workout
Knowing the purpose of the workout helps you train more specifically because you know what you’re trying to accomplish. To improve your acidosis threshold, do workouts at threshold pace (10-15 seconds per mile slower than 10K race pace or 20-25 seconds per mile slower than 5K race pace for most runners). To improve your VO2max and your ability to transport oxygen to your muscles, run intervals with work periods of 3 to 5 minutes (800 to 1,200 meters) at 1.5- to 2-mile race pace. To increase anaerobic endurance, run intervals with work periods of 45 to 90 seconds (300 to 500 meters) at mile race pace.
Don’t try to get faster by running the workouts faster than the pace at which you need to run to meet the purpose of the workout. Distance runners don’t do workouts to practice running faster. They do workouts to improve the physiological characteristics that will enable them to run faster in the future. As you progress, make the workouts harder by adding more reps, lengthening the distance of each work period, or decreasing the recovery intervals rather than by running faster. Only increase the pace of the work periods once your races have shown that you are indeed faster.