Whether you call it tempo running or anaerobic, lactate or ventilatory threshold training doesnt matter. Threshold training works, and adding it to your schedule is sure to make you faster and more efficient in endurance races.
Threshold training (running at your maximum aerobic steady state) is beneficial because youre working at what is supposed to be the optimal training intensity for endurance sport. If you work too hard, you risk too great a contribution of anaerobic metabolism to the energy supply, which leads to an accumulation of lactate and rapid fatigue: go too easy and you may not be training hard enough to gain the maximum benefit to your aerobic capacity. In short, there must be an element of control, which is where your heart rate monitor comes in.
Speed is often used to regulate threshold sessions, typically at between 10K and 10-mile pace. However, this can be hard to judge, especially when the terrain and/or environmental conditions vary from one session to the next. Whats more important is the intensity at which you run. You need to ensure that your body is receiving the appropriate physiological stresses.
Some athletes use the Conconi test to judge the correct heart rate, but this isnt perfect for everyone. Using percentages of maximal heart rate is another option, but the appropriate percentage does vary between individuals. In elite male middle- and long-distance runners tested in my lab, the average heart rate at threshold was as high as 93 per cent of MHR, compared to 85 per cent in cyclists. If you dont have access to a sports laboratory, your best bet is either to train at 85-90 per cent of MHR, or to select the heart rate that is associated with a pace 10 seconds per mile below 10K race pace.
Next you need to decide how to construct your threshold session. You could do a one-off, steady-state pace run for about 25 minutes, or add variety to your threshold sessions by splitting the work into reps. For example, start with 4 x 1M, and over a few weeks progress to 6 x 1M. The distance doesnt have to be exact (indeed, it may be more appropriate to do 4 x 6 minutes, etc), but always use two or three minutes of very light jogging to recover between reps. You can also play around with the sessions: other favourites include 3 x 8 minutes and 2 x 10 minutes.
All these sessions are at threshold intensity, governed by your heart rate. Start with a thorough warm-up and allow your heart rate to rise gradually in the first few minutes by adopting a sensible pace, so that you dont have to slow dramatically. You can assess your progress by seeing how far you get in your reps at the set heart rate. You will find not only that heart rate drops quicker in your recovery period as you get fitter, but also that you will maintain a faster pace throughout the session as you get used to threshold work.
So how do you fit these sessions into your schedule? It makes sense to vary the amount of threshold work that you do in the course of a week. Early in the year, one or two sessions totalling no more than 10 per cent of your mileage are sufficient. You can then step this up as you approach the racing season, so that 15 per cent of your work is at threshold intensity. If your volume of work increases through your schedule, then the total amount of threshold miles will also increase significantly in the course of the year.
As these sessions are quite tough, its important that the next session should be an easy one, in order to ensure that you reap the full benefit of your quality work.