I think we've got our thinking about exercise all screwed up. Instead of making time for exercise and planning rest days, we should make activity the norm and consider being sedentary a very unnatural state of being.
Our bodies are primed for activity. We're literally made for it, from our opposable thumbs to our unique muscle structure and capacity for endurance. We just fail to live up to our potential and modern living is making it all too easy - and tempting - to sit on our arses and get fat. The result, of course, is that we live most of our truncated lives in an unfit, unhealthy state until we die.
Wouldn't it be nicer to be healthy, strong, proud of your appearance and happier? Well, basically, that's the way it should be.
There are plenty of studies that show the crap we eat and our lack of activity contributes not only to heart disease, cancer and a multitude of other ills, it can cause premature aging, weak bones and weak muscles.
An active lifestyle is known to counter these things, and various studies have shown that exercise can not only help you sleep better and look better, it also releases chemicals such as endorphins that can make you feel better.
If you want to be fit to survive, you need to start using your body for what it was made for.