My experience, both as an athlete and as a coach, has repeatedly shown me that the MIND, in the constant search for improvement and perfection, often asks more than the BODY can give. 

Too often endurance athletes (cyclists, triathletes, marathon runners, cross-country skiers) give excessive importance to the total volume of hours/kilometers or the repeated execution of quality training, without regard to the need for adequate recovery and rest. Also in the case of imminent major competitions. 

If an athlete starts with another "loading" period before having completely recovered, his/her level of "fatigue" will grow from week to week, passing from a physiological overreaching to a pathological overtraining (see past articles on 53x12.com). 
Ideally, the optimum overreaching loading will be, after the appropriate "recovery", a lower feeling of fatigue in starting up with the next load. 

One of the first indicators of "FATIGUE ACCUMULATION" is the worsening of training performances, which is often difficult to discern and evaluate. 
Hence the importance of recording and analyzing the most crucial training sessions, with the help of power meters, heart rate monitors and stopwatches. 
For example, the execution of timed efforts at the same level of intensity is very useful: if a rider must reach a higher HR or a greater sense of fatigue to get the same performance, it may mean that the athlete is training too much. 
Paradoxically, even an unjustified increase in body weight can be a sign of fatigue, being in relation to the water retention (swollen legs feeling) that comes with overload and inflammation of muscle fibers. 

The obsession with continuity in training often leads to a decline in the quality of the same or of race results: the history of sports is full of great athletes who trained too much and got their best performances after a period of "forced recovery" due to unforeseen events or injuries.