As a runner, you are constantly confronted with a basic problem: there are literally dozens of different workouts to choose from, which differ dramatically from each other in nature, intensity, and duration. How is it possible to know which workout is the optimal one for a particular day?
Take tomorrow, for example: are you really sure that your planned workout is the absolute-best option? Might it not be preferable to carry out a more prolonged session at a lower average intensity? Would your fitness profit to a greater extent from a shorter, more intense effort? Should you add a bit of strength training to your day’s effort? Spend the day cross-training instead of engaging exclusively in your preferred sport?
The answers to these questions depend, of course, on what has gone before in your training schedule – and what you have planned for the future. Although single workouts can produce dramatic effects on fitness, they cannot be considered in isolation; they can synergize with other workouts in unique and positive ways, or they can ruin the progression to higher fitness because of the inappropriate way in which they are blended with their companion efforts. This unavoidable interdependence of training sessions means that it is seldom easy to answer the basic question about what to do on a given day.
Finding the answer is a bit less difficult, however, if you bear in mind a few principles about what workouts actually do and how they react to each other. The first principle is that variety is incredibly good for athletes. If you do fairly similar things in training over extended periods of time, your body will adapt to the challenge you are giving it in a way which is specific to the challenge – and then will adapt no further. Once your body can respond to a particular workout with relative physiological ease, it will stop changing and improving; it will cease its efforts to make your heart bigger or your stockpiles of intramuscular aerobic enzymes greater, and it will stop the process of fine-tuning your nervous system so that your nerves do a better job of controlling movement patterns at the intensities you have selected for your workouts.
Of course, you might say that all this is obvious – that everyone knows that training must be varied. If such awareness truly is universal, the gap between knowledge and action is a breathtaking chasm. In my years as a coach, I have noticed that the majority of athletes complete the same basic workouts month after month, year after year, and yet expect to achieve major breakthroughs in performance. When the breakthroughs do not occur (because the athletes’ bodies have adapted completely to the basic workout plan and have not been properly stimulated to tack on additional improvements), a typical response is to attempt to complete even more work, but of the same basic type. Unfortunately, this pursuit of volume at the expense of variety can often lead to overtraining and injury.