On the other side of the couch potato coin is the person who regularly exercises to the point of mental or physical exhaustion, and sometimes to overuse injury. A column in the New York Times addressed this recently and how top athletes must walk a narrow line that divides not enough training to reach peak performance from the risk of "overtraining syndrome." The article is worth reading.
That said, most seniors are not training for a spot on a national team, though some may still compete in age level competitions. So knowing when enough training is enough is just as important to them as it is to the young elite athlete. But for most of us who simply want to be strong and fit, knowing that we have not overdone it does not have to be such an exact science. A few practical guidelines will work just fine.
My own measure depends mostly on how I feel after a good night’s sleep. A few sore muscles now and then only means that I’ve changed a routine or pushed a little harder than usual, which is normal and to be expected. But a feeling of real fatigue after 7 or 8 hours of sleep on a good mattress usually means that I’ve been training too much or too hard. And should my fatigue last into a second day, a little time off is in order along with a reevaluation of my workout structure.
In other words, the cliche “listen to your body” happens to make a heck of a lot of sense