Many medical professionals follow the World Health Organization's recommendation of at least 150 minutes a week of moderate exercise or 75 minutes a week of vigorous exercise for adults.
However, their recommendation is only an arbitrary prescription because the exercise an individual need depends on many factors such as age, injury history, and other health concerns.
Regardless, no matter how unfit you are, even a little exercise goes a long way and it's better than none.

Myth: There is an optimal dose of exercise

The absence of physical activity makes us vulnerable to many illnesses, both physical and mental.
Exercising is not a magic bullet that guarantees good health for every person who chooses to exercise but doing so slows the rate of aging and substantially reduces your chances of getting a wide range of diseases.

Myth: Exercise is a magic bullet

Every diet benefits from including exercise into the routine.
You can lose more weight much faster through diet rather than exercise. However, for longer durations and higher intensities of exercise, it has been shown to promote gradual weight loss.

Myth: You can't lose weight walking

Running, walking, and other activities have been proven to keep knees healthy. Knees and other joints aren't car shock absorbers that wear out over time with use. If anything, you're less likely to develop knee osteoarthritis with running.
The main solution to avoiding knee pain is to learn how to run properly and train sensibly.

Myth: Running will wear out your knees

We all evolved to be grandparents to be able to provide food for our children and grandchildren.
Exercising and staying physically active as we grow older actually stimulates myriad repair and maintenance processes that keep our bodies groovy. Plenty of studies claim that exercise is healthier the older we get.

Myth: It's normal to be less active as we age

A lot of people don't like to exercise because they have natural tendencies to avoid it and it's hard to overcome.
To promote exercise we must make it a necessity and a rewarding activity. A way to accomplish this is to make exercise a social activity, where you'll be obliged to show up and you'll have fun but still be held accountable.

Myth: The 'Just do it' approach works

Let us not demonize normal behavior. It is true that too much physical inactivity is unhealthy, but sitting is not as bad as people make it out to be. It is only leisure-time sitting that is strongly associated with negative health outcomes rather than work-time sitting.
Those who sit actively by getting up every other 15 minutes and so wake up their metabolisms and enjoy better long-term health than those who don't.

Myth: Sitting is the new smoking

Our ancestors' lives were certainly not easy, but they were not super-strong or super-fast. Most of them were reasonably fit for hunter-gatherers, but they were moderately strong and not especially fast.
On the average our ancestors only spent 2 to 3 hours a day doing moderate-to-vigorous physical activities.

Myth: Our ancestors were hard-working, strong, and fast

Our ancestors evolved to avoid needless exertion so don't feel too bad about the natural instincts we still carry. Learn to accept that it is a natural behavior and that it is difficult to overcome it.
Physical activity costs us calories and up until recently, we were always short of supply. Back in time, food was limited and every calorie spent on physical activity was a calorie not spend on critical functions like body maintenance, storing energy, or even reproducing.

Myth: Avoiding exertion means you are lazy

Whenever you move to do stuff, that's physical activity. Exercise, however, is a voluntary physical activity that one undertakes for the sake of fitness.
Exercise is a modern behavior. Humans, for many ages, were only physically active when it was necessary or when it's rewarding. Gathering food among other survival activities are considered necessary while playing, dancing, and developing skills, are rewarding.

Myth: It's normal to exercise

We all believe we should exercise more. So why is it so hard to keep it up? Daniel E Lieberman, Harvard professor of evolutionary biology, explodes the most common and unhelpful workout myths