Once upon a time, I saw a short, proper and single word for "lack of exercise", but I forgot it.
A lifestyle devoid of physical activity is usually described as sedentary, as for example in this Wikipedia article on Sedentary Lifestyle.
I think inactivity may suggest the idea:
- the state of being inactive , idle or inert.
UNHEALTHY DIETS & PHYSICAL INACTIVITY
- Unhealthy diets (especially those which have a high content in fats, free sugars and salt) and physical inactivity are among some of the leading causes of noncommunicable diseases (NCDs) including cardiovascular diseases (CVD).
Source: www.who.int/nmh/publications (World Health Organization)
According to ODO:
sed·en·tar·y ˈsednˌterē/Submit adjective (of a person) tending to spend much time seated; somewhat inactive. synonyms: sitting, seated, desk-bound, stationary; More (of work or a way of life) characterized by much sitting and little physical exercise. synonyms: sitting, seated, desk-bound, stationary; More (of a position) sitting; seated. synonyms: sitting, seated, desk-bound, stationary; More
How about flab?
The condition of being flabby: Most of the new recruits had run to flab in civilian life.
flabby: having flabby flesh, esp. through being overweight Dictionary.com
The condition of being out of shape
out of shape and out of condition: not in good physical condition. I get out of breath when I run because I'm out of shape. Keep exercising regularly, or you'll get out of condition. McGraw-Hill Dictionary of American Idioms and Phrasal Verbs
Weight in excess of what is considered normal, proper, or healthful Random House
The condition of being obese; increased body weight caused by excessive accumulation of fat. FOD