A tempo run is a sustained effort at lactate threshold intensity, which is the fastest pace that can be sustained for one hour in highly fit runners and the fastest pace that can be sustained for 20 minutes in less fit runners. Tempo or threshold runs serve to increase the speed you can sustain for a prolonged period of time and to increase the time you can sustain that relatively fast pace.
Example: 1 mile of easy jogging (warmup) + 4 miles at lactate threshold pace + 1 mile of easy jogging (cooldown)
There is a specific type of tempo run that is known as a marathon-pace run. A prolonged run at marathon pace is a good workout to perform at a very challenging level in the final weeks of preparation for a marathon, after you’ve established adequate raw endurance with long runs and longer progression runs featuring smaller amounts of marathon-pace running.
Example: 2 miles at natural pace + 13.1 miles at marathon pace