Many runners, as they become more advanced and comfortable with their training, begin to wonder how fast they should run on a day-to-day basis, usually in comparison to their race paces. Even more advanced runners struggle with the idea of what constitutes an "easy day," a "medium-hard day," and so on. And everyone who lines up to race - or simply seeks speed improvement for strictly personal reasons - wonders what specific workouts should be done to bring this improvement about, how often they should be performed, and how fast they should be run.
In sharing ideas with a number of other coaches it seems that a high fraction of them have, quite by accident, concurrently come up with a notion I'll call "The Rule of Five." This means that leaving aside your ordinary, garden-variety everyday "easy" run, there are five basic pace zones you should strive to reach regularly in order to gain the most benefit from your workouts. These paces all emphasize a different physiological aspect of performance. They are:
1. Short speed. These are bursts of faster running lasting from about 30 seconds to no more than about 1:30 (often 200 meters to 400 meters), interspersed with walk or jog breaks lasting between about one or two minutes. Any one workout might include between eight and twelve repeats. These are not quite sprints, but are run at a pace much faster than the runner will average in any race. They teach the body to develop proper mechanics ("form") at high speeds, emphasize certain fuel-burning processes in "fast-twitch" muscle fibers, and - as much from a psychological standpoint as from a physical one - train the runner to "kick" at the end of a race (i.e., finish fast).
2. Medium speed. These are stretches of faster running held for about two to five minutes, or roughly 400 meters to 1200 meters. These are run at roughly the pace you could hold for ten or twelve minutes going all-out (faster than 5K race pace and about equal to two-mile race pace). Physiologists have determined that this speed is best for teaching the body to develop its ability to use oxygen most efficiently, since it's somewhere on the border of "aerobic" and "anaerobic." A half-dozen repeats is a good number to shoot for and the rest interval should be about 75% of the work interval (e.g., if you are running 6 x 800 meters in four minutes than you should walk/jog for three minutes in between.
3. Long speed. These are stretches of running of five to seven minutes and are done at roughly 5K race pace. They offer many of the same benefits as medium-speed workouts but train the body to sustain a hard pace for longer periods of time. The rest interval should be about half the repeat duration and the total amount of hard running should be around 15-20 minutes (i.e., about three repeats in all).
4. "Tempo" pace. This is the pace you could hold for a race lasting right around one hour. Scientists have determined that this is the speed at which lactic acid - a by-product of "anaerobic" running - begins to build up in the bloodstream faster than the body can clear it. By running at this pace, you train your body to handle and clear lactic acid more efficiently. This is an especially valuable training pace for marathoners. Unlike the other workouts mentioned, tempo workouts are done in one continuous run of 20-25 minutes. If you don't know how fast to run, think "comfortably hard" - too fast to talk, not so fast that it really has you sucking wind.
5. Marathon pace. Not everyone is a marathoner, but longer runs at a fairly high intensity train the body to use glycogen - its primary fuel source - more efficiently because they tax the glycogen in "slow-twitch" muscle fibers preferentially. Fitting in an hour of marathon-pace running at the end of an hour and a half run is an example of an "MP run." These are not as intense as the other workouts but they will tire you for a few days afterward. Take care to stay hydrated and fueled!
So that's it. In terms of workout frequency, it's best to do one or at most two of these sessions a week, so that the body has a chance to recover for a few days between hard efforts. Don't do any of these on back-to-back days and make sure you've done at least ten minutes of light jogging before starting a speed workout to minimize injury risk and make sure your gears are operating smoothly. Ideally, your schedule would see you rotate through these different workout types, so that you're never going more than a couple of weeks without touching on any one of them.
People with less than about six months of running under their belts are advised to wait until they're sufficiently conditioned before undertaking these efforts. Just keep it fun - not that the "Rule of Five" isn't a fun concept in itself!