The effects of de-training and aging.

The bottom line, as usual, is use it or loose it. When you stop training or as you age, it is good to regularly remind your body that you need it. These excerpts are from George Dvorsky's article This is What Happens to Your Body When You Stop Exercising:

What was once a 20-minute 5K has now turned into a 23-minute 5K. By the 1- to 2-month mark, athletes typically lose about 19-20% of the VO2, and a significant decrease in muscle power. At this stage, they’re pushing 24-minutes or so. After two months, athletes typically exhibit a 26% reduction in VO2.

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“Certainly, it will eventually change with age, but what typically changes are the habits of practicing at high level intensity, and that’s how you slow down,” he says. “It’s not so much the frequency of exercise, but the quality of exercise.”
...“Physiologic and performance measures improve rapidly during childhood and achieve a maximum between late adolescence and approximately age 30,” says Bergdahl. “The decline starts shortly after—sometimes after 40 depending on the body system—and the changes are similar to detraining.”...A lack of exercise has been shown produce certain psychological effects, including depression and lower self-esteem. ...“Part of that has to do with blood flow (again oxygen) to the brain,” explains Bergdahl. “More oxygen equals better brain function. ...Pino recommends that athletes, even after running a marathon, or when nursing an injury, can still find ways to remain active. He recommends such things as the stationary bicycle, elliptical, or rower. Many of his athletes cross-train during their detraining phases.
“This gives the muscles that work hard a break,” he says, “but they won’t lose that much aerobic fitness.”