The tempo run is the the second of the 3 weekly runs prescribed by the FIRST 'Run Less, Run Faster' training program - the other two being interval training and the long run .A tempo run is also known as a lactate-threshold, LT, or Anaerobic threshold, AT or just threshold run. The tempo run pace is 'comfortably hard' - slower than the pace of interval runs and faster than a long run pace. One rule of thumb is about 15 to 45s slower than 5K race pace depending on the distance of the tempo run. The RLRF program specifies the pace for each of the 15 tempo tempo runs depending on the target marathon pace.
LT is the point at which lactic acid (a by-product of glucose metabolization) begins to accumulate in muscles. An accumulation of lactic acid in the muscles leads to the fatigue and soreness that runners experience when running hard. Increasing the LT by doing tempo run can reduce the accumulation of lactic acid which results in an ability to run faster without suffering muscle fatigue. Tempo runs are also helpful for developing the mental toughness and stamina needed for racing.
The lactate-threshold or anaerobic threshold run is for about 40-60 minutes duration at an intensity at which lactic acid accumulates in the blood stream faster than it can be cleared away - this is usually at 80-95% of maximum heart rate. Increasing the anaerobic threshold allows the body to run at faster and faster speeds before fatigue and lactic acid take over.Maintaining a specific and consistent pace is the most important aspect of a tempo run. The RLRF program specifies tempo runs of 3 paces - short-tempo pace for 5K distance, mid-tempo pace for up to about 8K and log-tempo pace for distances up to about 10K.
The tempo interval is a variation which are tempo runs interspersed at regular (say, 2Km or 10-minute) intervals by 30- to 60-second rest periods. This pattern diminishes the psychological difficulty of the workout while preserving the aerobic benefits, allows greater volume and may help guard against excessive speed.
Personally, of the 3 workouts prescribed by the RLRF I used to enjoy the tempo run the most - medium distance at medium intensity (intervals being short distances at very high intensity/effort and long run being long distances at relatively lower intensity) . However, in the current training cycle for the Bangalore marathon I have struggled a bit with the tempo runs specially the mid-tempo pace run - missing the target pace by a few seconds for some of the Kms.