This is what I mean by variety: just when your body is getting pretty smug about how fit it is getting, you pull the rug out from under it and give it something uniquely challenging. Instead of running the same hill at your best pace, for example, you turn on the heat with very short hill repetitions. Instead of focusing completely on the repetitive movements associated with running, you embark on an intense six-week program of whole-body proprioceptive work to improve your overall coordination and efficiency. Or, instead of relying on ‘tempo’ workouts to lift your lactate threshold, you substitute much shorter – but more intense – intervals to give your lactate-processing abilities a boost. Overall, you prod your body into continuing the improvement process instead of resting on its laurels.
Note, however, that using variety in your training does not always mean making your workouts tougher; in fact, sometimes you should make them easier. Instead of your usual 60-minute run, you might go out for just 30 minutes; and instead of training for eight hours per week, you might cut back to four. Again, you are prodding your body into continuing its improvement, but this time you are doing so by letting it recover from the hard work you have done. In effect, the training stimulus is drastically reduced, but the stimulus to synthesize new things within your neuromuscular and cardiovascular systems is accentuated. A period of 5-7 days of very light training often does a great job of augmenting fitness; and you can add these periods of lightness to your training every 3-5 weeks, or whenever sluggishness and fatigue hang around for more than a day or so.