Tempo running is one of the cornerstones of long term aerobic development. What is a tempo run? Well, that term is typically applied to runs that are are somewhat fast but not a sprint. It is typically used to describe workouts that include only one interval or repetitions, without repeating anything. That is to say a continuous run at a somewhat fast pace. This includes just about every run from a few miles up to distances over a marathon, which are at close to marathon pace. Tempos are sometimes referred to as quality work performed at slower than race pace.
The general purposes of tempo runs are lactic acid production and metabolism, aerobic respiration, and mental endurance. In my opinion, and based on what I have read from Canova and Daniels, tempo runs accomplish these purposes better than any other workout.
Below is a graph of my tempo runs over the past four years. Now this graph is incomplete and a little misleading because I spent many workouts running tempo paces over long intervals (such as 3 x 1.5 mile, or 5 x 1 mile). Which means I was training the aerobic and lactic acid systems in my body through interval training, but I was not doing it continuously. Since ending my NCAA elegibility in 2009 I have spent more time doing tempos, in large part because of a lack of training partners, but also for their benefits.
For the sake of comparison the graph below is all of my interval runs during the same time that were at least 3 miles or longer. However, these workouts include jogging between intervals so a four mile workout could be 1 mile fast, .5 mile slow, 1 mile fast, .5 mile slow, 1 mile fast. So 120 miles of interval workouts is not 120 miles of quality fast running.
One the workouts that I try to do regularly now but never really did in college, is a longer tempo run. Often seven to ten miles. Ideally, before I am comfortable running a marathon I will have at least one tempo workout over 20 miles in length. These slower tempos do not produce the immediate results that faster interval workouts do, but they contribute to making me stronger and generally faster. I notice nearly every time after a 7+ mile run that I feel faster for a few days in my other runs.
However, most of the above information aside, one of the most potent workouts in the history of sport is the 20 minute tempo run. It is run at a pace that is about the pace the runner can handle for one hour. It corresponds to a lactic acid concentration of about 4 mmol in the blood. Workouts of this duration are so widespread that I would guess every runner who runs 1500 meters or farther in the Olympics or Olympic Trials has done 20 minute tempos. Even Lance Armstrong does these workouts on a bicycle.
The purpose of a 20 minute tempo is to run very close to the limit of sustainable lactic acid consumption, but not faster, so that the body adapts to the production and consumption of lactic acid and can then run marginally faster at the limit of sustainable lactic acid consumption.
For example, anaerobically runner A and runner B can both tolerate a lactic acid level of 15 mmol in their blood before they slow to a jog. Runner A can maintain a pace of 5:30 per mile for a blood lactic acid level of 4 mmol. Runner B can only run 5:40 per mile for a blood lactic acid level of 4 mmol. Pretend that at a pace of 30 seconds per mile faster than the sustainable lactic acid consumption rate both runners produce 5 mmol of lactic acid that can not be metabolized every mile. So at a pace of 30 seconds per mile faster than the blood lactate level of 4 mmol corresponding to sustainable lactic acid metabolism both runners can run three miles before being slowed to a job. In a race runner A will win because all other things being equal he has the ability to metabolize the lactic acid that he produces at a faster pace than runner B.
It is such a great, objective workout that I feel it is one of the best ways to judge fitness outside of races and time trials which come with long recovery times, an inevitable short term setback.