If you're thrown by illness or whatever, this is it
http://www.washingtonpost.com/lifestyle/wellness/how-long-does-it-take-to-get-out-of-shape/2015/04/14/1e37a764-dcbd-11e4-a500-1c5bb1d8ff6a_story.html
"The first thing to slide is your aerobic fitness. After 10 to 14 days with little or no physical activity, the body’s ability to effectively consume and use oxygen, sometimes referred to as VO2 max, begins to decline... Studies show “notable reductions in VO2 max within two to four weeks of non-training, mostly due to decreased cardiac output and decreased blood volume,”
...
"Muscles
Detraining has a less immediately dramatic impact on muscular strength and endurance. During the first few weeks off, the effects are slight, Matthews says. After about four weeks off, however, muscle fibers begin to shrink, and sometime between then and eight weeks, that decline becomes measurable, Zimmerman says. Your muscles will feel less firm; those six-pack abs might start to sag a bit. But this, too, is reversible."
....
"How do you stop the slide? The answer is both harsh and obvious: Try not to stop exercising in the first place. Of course, if you are seriously injured or very ill, by all means rest. (And remember, too, that rest and recovery are a vital part of any exercise regimen.)
But there are a number of exercises you can do that are less taxing to your body and schedule. If you can’t run, you might bike or swim or even just walk. Add stairs to your daily routine. Or if you are able, do squats or light weightlifting, even using common household products, such as cans of food. If that’s too strenuous, try a chair workout."
