For me, it's not how hard I work out, but how well I can relax later. There's definitely a big place for rest and relaxation in health and fitness. Don't work out vigorously and then lie down. Instead, cool down, do some stretching, then sit and rest. As long as you do 30 minutes of aerobic activity as many days a week as possible, you get the benefit of your activity.
Rest is even more important for elite athletes, because the body needs that time to recover.
Probably the thing that I have going for me more than most athletes is I'm able to rest. If you don't wear yourself out, you can exercise more later on. Resting is as important as the exercise. When I'm training, I work out three or four times a day. I start with two hours of running, then I rest.