There are some basic exercise principles that determine what workouts are more effective than others, and many of these principles are merely stating the obvious: for example, that all humans are subject to the same basic rules of cardiovascular operation and muscular function. Because we as humans do not have the energy system of something with an exoskeleton, for example, we can focus on the operation of our bodies as warm-blooded mammals. Though the science of exercise can get very technical, the basic idea is that there are two primary energy systems in the body, and these are referred to as aerobic and anaerobic.
The various unique running workouts focus on engaging these two systems differently, but before we jump into those workouts, let's examine how the body operates in various race distances. The 60 meter and 100 meter dashes are purely anaerobic exercises; they engage fast twitch muscle systems and temporary energy sources (think of this as hitting the boost button for a dose of Nitrous Oxide in a sports car). The 200 meter is almost completely anaerobic, but some runners hit aerobic stages in the final strait of that race.
The 400 meter is a unique animal, because it is a juggle between the anaerobic and aerobic energy systems. Michael Johnson, who still holds the world record in the 400m, showed mastery over the two energy systems through his dominance in the event. The 400 meter dash utilizes between 40-75% anaerobic energy, and the remaining comes from the aerobic systems. After this point, each successive increase in distance means a rapid decrease in the percentage of anaerobic energy used in the race.
Running workouts focus on utilizing these two systems to increase endurance. To improve speed and endurance, many runners of varying disciplines rely on 400 meter repeats. Taken alone, the 400 meter is a sprint event, but when coupled with 8 to 25 repeats, the 400 meter turns into an extremely dynamic distance test. High school, college, and professional athletes all use a form of this repeat workout, as do soldiers of the U.S. military (the Army version of this is referred to as 60-120s).
The operation of energy systems is relevant, because doing a workout like 400 meter repeats works both anaerobic and aerobic endurance. Repeated 50-70 second bursts of energy are extremely intense, and muscular soreness in connection with this exercise is entirely normal. Another good thing about 400 meter repeats is that they engage the muscles differently. In a distance race, the quadriceps muscles are the most utilized, but sprinting shifts some of the load to the calves, and builds strength in a new muscle group.
You should focus on maintaining pace when suiting up for a 400 meter workout. Pick a pace that is appreciably faster than your race pace, but slower than your maximum speed. If you are training for 1600 meters or 3200 meters, for example, you should aim for right in the middle of your max 400m time and personal best. For example, in my junior year of high school, my best two mile time was right around 12:00, and my fastest 400 meter was 1:00, so I ran my repeats between 1:15 and 1:17. My best mile time was 5:25, however, and I would drop the number of repeats from 8 to 4-6 and drop the time from 1:15 to 1:10 or so.
Once you establish your pace goal, recognize that the repeats are going to become increasingly more difficult as you try to maintain pace. You can adjust to this by pushing even harder; your repeats should mimic the given stages of your race. Thus, if training for 3200 meters or two miles, your last two repeats should reflect an increased pace and kick. Furthermore, you should attempt to fight the natural tendency of the body to fade in the middle laps: laps 4-6.
When presented at once, the 400 meter workout sounds rather complicated, but it is in fact very simple. Most ordinary tracks are 400 meters per one lap, which makes them easy to use for training. With practice, the workout becomes increasingly easier to perform.