Tempo runs are fast pace efforts that are brisker than long run pace, but not so fast that you are too tired for your next run. The objective is to twofold: 1) raise your anaerobic threshold so you can race faster; and, 2) help you sharpen up for races by running at near race pace (event specific pacing).
Your anaerobic threshold is the point where you shift from running primarily using the aerobic system to a greater percent of anaerobic running. Anaerobic running places you in oxygen debt and can only be maintained for a short period before lactic acid slows you down. Try sprinting 800m and you will find out how this feels. Improving your anaerobic threshold guarantees you will be faster racing distances from 5km to 42.2km.
How fast is your anaerobic threshold? It is a little slower than your 10k race pace (about 4% slower.) It also corresponds closely to your 15km race pace. A 50 minute 10km runner races at 5min per km and would run their tempo pace at 5min per km plus 4% = 5min per km x 1.04 = 5:12 per km.
Only a 12 second difference between 10km pace and 15km pace in the example above. It appears there is not a major difference in the speed but there is a big difference on how your body responds. The anaerobic threshold is at the inflection point where lactic acid accumulates very quickly at a slightly faster pace. You need more recovery time between workouts when running 30 minutes at 10km pace versus 15km pace.
The most effective training speed to improve your anaerobic threshold is to run at your anaerobic threshold speed but you can run slower up to 10 to 15 seconds per km and continue to enjoy some benefit. On the training schedule there is a weekly tempo workout each Monday. Rather than a continuous run of 30 minutes I break the tempo workouts into long repeats of 5 to 15minutes that allow you to maintain a high quality pace. A good example of this kind of tempo workout is 3×10 minutes with a 2 minute rest. The 10 minute repeats are run at different paces; ie 10 minutes at marathon pace; 10 minutes at half marathon pace and 10 minutes at 15km race pace. The first 10 minute repeat at marathon pace allows you to warm-up. The rests are short and do not allow for complete recovery between the repeats. This helps boost the anaerobic threshold and offers an opportunity to practice event specific pacing.
Tempo runs are part of your regular weekly routine and are used as fitness building blocks. Also, tempo workouts help you sharpen up for races by activating your muscles for a fast race pace, and by improving your ability to concentrate. The workout is more effective if you do not run faster than your 15km race pace. If you find the workout too easy then shorten the rest or speed up the rest jog but do not increase the pace of the repeat faster than 15km pace. The key is not to max-out so you finish feeling sharper and fresh for the Wednesday speed workout
You can expect to run faster over time when you control your pace. As the weeks pass you will become fitter and should be able to increase your tempo pace without accumulating too much lactic acid.