High-performance training demands risk taking. The typical risks come in the form of breakdowns such as overtraining, injury, and illness. Mostly moderately hard workouts are what you should be seeking in training to avoid these common breakdowns. That means balancing stress and rest. This balancing in order to produce high fitness is very similar to growing a flower. With the right amount of nutrients in the soil and just enough watering the flower will grow and bloom. Too much of either of these otherwise good things and the plant withers and fails to achieve its potential. It’s the same way with using training stress to grow your fitness. Too much and you breakdown.
Getting the right amount of stress in your training comes down to experience. There is no one-size-fits-all formula for determining how long, frequent, or intense your workouts should be. That is something that must be determined over time. It’s highly individualized. You will know when you’ve done too much in a workout or a closely spaced series of workouts. The most common symptom is that your recovery will be slow taking more than about 36 hours. If you are still fatigued 48 hours after a workout you could assume the preceding training was not moderate. In this situation “fatigue” means you are unable to repeat the workout or another such challenging session. You’ve achieved stage 2—functional overreaching—and may even be on the way to stage 4 overtraining. Of course, over time, as your fitness increases with patient and non-greedy training, what was once an overly hard stage 2 workout becomes a moderate stage 1.
There are times when an experienced athlete may decide to take a risk by doing a stage 2 workout or even several back-to-back. But the risk must be carefully calculated and fatigue closely watched. Avoiding injury, illness, and overtraining at such times still requires some degree of moderation. If you push your body just a bit too much for even a few days your season could come crashing down. Patience is always necessary, especially with such risky training.
On the other hand, if you aren't tired after two or three weeks of training in the base or build periods then you aren't training hard enough. The risk is much too low to realize a reward. In other words, your training is too easy. Fitness and fatigue trend the same direction. If your fatigue is increasing then you are becoming more fit. If fatigue is never produced then neither is fitness. You must frequently become moderately tired if you are to become fitter. So fatigue is a good thing, not something to be totally avoided. The only issue at hand here is how long the fatigue lasts.
Use your fatigue wisely. Don't waste it on workouts that don't fit your unique needs. Train with a purpose.