"Tempo" is a term used by runners all over to describe a variety of different workouts. Some say that a true "tempo" run never crosses over the lactate threshold. Others have a different specific definition of a tempo run. Still others, including myself, use the word tempo as a more broad term. A "tempo" workout to me is any workout that involves sustaining a faster pace over a longer period of time. This could be a 4-6 mile run at half marathon pace, long intervals like 2 or 3 mile repeats, or a longer sustained run at or around marathon pace.
The main purpose of a tempo run is to run at a comfortably uncomfortable pace for an extended period of time. The length of a tempo run depends on the pace and your body's ability to sustain that pace. Running at a faster pace for a sustained period of time develops your body physiologically in ways that intervals and easy running do not. When you run, your muscles produce lactic acid as a byproduct of exerting energy. If you simply continue to run at an easy pace, your muscles are able to efficiently clear the acid from your muscles. If you run faster, your muscles are required to exert more power and use more energy resulting in more lactic acid byproduct. Everyone's body has a limit or a "red line" known as their lactate threshold. This is the point at which lactic acid is being produced faster than your muscles are able to clear it and it begins to accumulate. Accumulation of lactic acid creates fatigue and soreness in muscles. By practicing paces just under, right at, and just over your lactate threshold helps your muscles to practice clearing the acid at a higher rate and remaining in an aerobic zone at faster paces. This is a key component of fitness for any distance from 5k- marathon.
I believe that running slightly below your lactate threshold is not the most effective way to raise the threshold, but it is better to run at a variety of paces and distances at, below, and above the threshold for a well-rounded approach and full benefit from training.
This is the "bread and butter" of tempo workouts. One sustained run at an even pace that toes the red line but doesn't step over. Typically, a run like this is about 30 minutes in duration and run at a pace you can race for one hour. For advanced to elite runners, this is close to half marathon pace, whereas for beginning runners this is closer to their 10k pace. This pace will not feel easy at the start, but it will not instantly get harder. The feel is often described as "comfortably uncomfortable." As the run progresses, you'll begin to fatigue, but if the pace is right, you'll be able to maintain an even pace without going all out and you'll end with something still left in the tank.
A tempo workout that stays below the lactate threshold level is often referred to as a "steady state" run, mainly because you don't quite flirt with crossing the red line like you do in a traditional tempo run mentioned above. Your body is just far enough from the red line that it can comfortably keep up with the effort level for a longer period of time without significant fatigue or lactic acid build-up. In training for longer races, I may do a steady state run anywhere from 8 to 16 miles, depending on where I am in the training segment. After a break, I like to do at least one "steady" run before I dive into tougher tempo workouts again. A "steady state" pace for an advanced to elite runner may be at or slightly slower than marathon pace, depending on fitness level and maturity of the training segment. For a beginner, a steady state run will often be faster than marathon pace and closer to half marathon pace. Also, a beginning or intermediate runner will probably not be able to run 16 miles at a "steady state" just below their threshold, but they may be able to run the same amount of time at a steady state as it would take a more advanced runner to run 16 miles.
No one can run above their threshold for very long. No matter who you are or how fit you are, when you step over the red line, you're over. Lactic acid build up in your muscles and you fatigue rapidly. So, you won't be doing 30 minute runs over your lactate threshold (unless you plan on significantly slowing down...). As I mentioned at the very beginning, many runners will claim that a "tempo" run never crosses the red line and any run that does is no longer working on raising the lactate threshold level. I believe just the opposite and regardless of what system in the body I'm actually working, I've seen fitness benefits in myself as well as others around me by pushing past the threshold line in a workout. I believe that you MUST push past the line to really see that line move! This can be done very effectively with long intervals. I utilize long intervals a LOT in my training especially for the marathon. To race a half or especially a full marathon, threshold levels must be high and a fast pace must feel comfortable for a long time, therefore sustaining faster paces must be practiced a lot in training. In training for longer races, I've done 4x2 mile, 3x3 mile, 2x4 mile, 3-2-1 mile, and plenty of other variations. Long intervals allow you to run higher volume workouts above the threshold level. Additionally, for shorter distances, a short and fast tempo can be done at the beginning or within a speed workout to help work multiple systems in a single workout. Ex. 2 mile tempo + 4x400.