From this NTY article Why Trainers Say, 'Slow Down':
"Of the tens of thousands of Americans who pay as much as $180 to register for marathons, as many as 25% fail to make it to the race. Injury, illness and loss of motivation as a result of overtraining are major reasons for this."
"No matter how conclusively science may prove the value of rest and recovery, the culture of endurance sports lionizes those who seemingly never rest."
"The body responds beautifully to the right schedule of training stresses," Lynn Bjorklund, who in 1981 set the still-standing female course record for the Pikes Peak Marathon, wrote in an email. "However, too much stress and not enough nutrition or recovery pushes your body toward injury and illness. You need to stay in that zone of just enough, and that takes a very high tuned and honest appraisal of yourself."