One of the biggest mistakes runners make is running either too fast or too slow for their workouts.
The goal of training is to obtain the greatest benefit while incurring the least amount of stress, so you should run as slow as you can to meet the purpose of the workout. From slower to faster, here are the only four speeds at which you need to train.
Easy Runs and Long Runs
The purpose of easy and long runs is to stimulate the physiological, biochemical and molecular adaptations needed for good endurance. Some of those adaptations include the storage of more fuel (glycogen) in your muscles, an increased use of intramuscular fat at the same speed to spare glycogen, a greater number of red blood cells and hemoglobin, a greater capillary network for a more rapid diffusion of oxygen into the muscles and, through the complex activation of gene expression, an increased mitochondrial density and number of aerobic enzymes to enhance your aerobic metabolic capacity.
Since many of these adaptations are volume-dependent, the speed of easy runs is not as important as their duration. Out of all the workouts you do, the exact pace matters least for easy and long runs.
The single biggest mistake competitive runners make is running too fast on their easy days. By doing so, you add unnecessary stress to your legs without any extra benefit and you won't be able to run as much quality on your harder days.
More: 3 Rules for Easy Runs
Do your easy runs at about 1? to 2 minutes per mile slower than your current 5K race pace (about 70 to 75 percent maximum heart rate). As you increase your weekly mileage, you may need to run slower to accommodate the extra volume. Speed-type runners (those who fare better at shorter races) will have a greater difference between their race pace and easy running pace compared to endurance-type runners (those who fare better at longer races).
Acidosis Threshold Runs
The acidosis threshold (AT) marks the transition between running that is purely aerobic and running that includes significant oxygen-independent (anaerobic) metabolism and the development of acidosis.
AT is the fastest speed that you can sustain aerobically. The purpose of AT training is to increase the speed at which your AT occurs, which allows you to run faster before anaerobic metabolism and fatigue begin to play a significant role.
The AT workout (often called a tempo run) is one of the most difficult for people to run at the correct speed since it requires holding back and not pushing the pace. There's a comfortably hard feeling to the pace that requires practice.
For recreational runners, AT pace is about 10 to 15 seconds per mile slower than 5K race pace (or about 10K race pace; 75 to 80 percent max heart rate). For talented and highly trained runners, it's about 25 to 30 seconds per mile slower than 5K race pace (or about 15 to 20 seconds per mile slower than 10K race pace; 85 to 90 percent max heart rate). The better your endurance, the longer you can hold your AT pace and the better you'll be at sustaining any percentage of that pace.
VO2max is the maximum volume of oxygen your muscles can consume per minute. The purpose of VO2max intervals is to increase your VO2max by running at the speed at which VO2max occurs. This VO2max pace corresponds to the pace at which you reach your maximum heart rate, maximum stroke volume (volume of blood the heart pumps per beat) and maximum cardiac output (volume of blood the heart pumps per minute). Cardiac output equals heart rate times stroke volume.
In lieu of a laboratory test to determine your VO2max pace, either run at close to your maximum heart rate or use your current race performances.