Fatigue…a hallmark of overtraining, which is best described as decreased work  capacity resulting from an imbalance between training and rest. Yes, one must travel close to the “valley of fatigue” to optimize recovery and adaptation. The idea is to go to the edge and then back off. Plunging into the valley is not desirable and causes the overtraining indicators to to rear their ugly head. Smart training is knowing where your edge is. In addition, older athletes require more rest/recovery than younger athletes in the training cycle.