Training + Rest = Fitness (Form)
Most endurance athletes are very good at the training portion of this equation. But without carefully planned rest days, weeks and months, an athlete's fitness will plateau (best case scenario) or plummet (when over-trained).
Many endurance athletes don't rest enough. It can be intoxicating to keep training longer/faster, etc. And for a while, it can work. But eventually, it will fail. Sometimes the athlete simply never again improves - a subtle failure that they may not even recognize. Most times, it will result in repeated illnesses or injuries. Sometimes, the athlete can reach a state of over-training - a little understood scrambling of their body's endocrine and energy systems that can take months or years to recover from.
As we age, the need for rest becomes more obvious. But, the younger athletes would be ahead of the curve to recognize that they need rest as well.
Without rest, you will not attain your potential.
Before we had kids, my husband and I would train until we dropped each weekend morning. Then, we would typically eat a huge lunch and hang out sleepily on the couch for a few hours until we had grabbed back some strength. That stopped abruptly more than 10 years ago with the birth of my daughter. There went most of our naturally built in recovery hours each week. We had to learn to listen to our exhausted parental bodies and skip workouts when necessary. Without those couch hours each week, we couldn't train until we dropped.
I figure that in about 10 more years, we'll get back our couch and more training energy. We may be 20 years older then, but I bet we'll find some revival of our old energy when we're not chasing kids 24/7. (We'll also miss them terribly, I'm sure. These years with kids are completely wonderful!)
Yesterday, I felt exhausted from 3 tough weeks of training. I managed to sit down and watch the Olympics for several hours - a minor miracle. Boy, did I feel good today!