Recognizing the various health benefits associated with a smart exercise habit isn't new. The great Athenian philosopher Plato recognized the value of "movement and methodical exercise" some 2400 years ago, and 350 years before Plato was born, the ancient Greeks began holding their Olympic Games, which ran from 776 BC to 394 AD. 
NOT ONLY ABOUT LOSING FAT & BUILDING MUSCLE
Exercise science has come a long way since 776 BC. We now know that exercise, when performed smartly, carries with it several benefits, including reduction in…
* chronic pain
* cardiovascular disease & hypertension risk
* diabetes risk
* cancer risk
* osteoporosis risk and
* depression risk.
We also know that a smart exercise habit has been shown to improve...
* cardiac & respiratory functioning
* bone density
* sex drive/libido and
* overall appearance.
Exercise has also been shown to be an effective stress reliever, a way to strengthen social bonds, an excuse to get out in nature (which carries its own benefits), and not to mention... exercise CAN be fun! [2-3]
HOW EXERCISE CAN HURT YOU...
Of course exercise can have adverse effects too, especially when dangerous and/or inappropriate activities are performed. Some of the risks associated with exercise (especially certain types of exercise) include…
* increased risk of injury
* trouble breathing (which can be dangerous for asthmatics and those with respiratory conditions)
* exacerbated joint or soft tissue pain (especially for arthritics and those recovering from injury)
* heat stroke/dehydration/electrolyte imbalance
* excess free radical production and/or dangerous increase in metabolic by-products (both of which can damage cells and tissues over time)
* rhabdomyolysis (the breakdown of muscle tissue)
* accelerated aging
* positional vertigo and
* cardiovascular risks (including heart attack or stroke). [4-5]
Despite these risks, plenty of research shows that a certain amount of exercise is generally associated with a reduced risk of early death. 
So then the question becomes...
HOW MUCH EXERCISE DO WE NEED TO
AVOID AN EARLY DEATH & MAXIMIZE LIFESPAN?
A very large 2015 JAMA study looked at data from more than 660,000 participants and found the following…
* Those who didn’t exercise had the highest risk of early death (no shocker there)
* Those who exercised around 150 minutes a week (equivalent to 30 minutes a day, 5 days a week) reduced their risk of early death by 20%
* Those who exercised 150-300 minutes a week (equivalent to 30-60 minutes a day, 5 days a week) reduced their mortality risk by 31%
* Those who exercised 300-450 minutes a week (equivalent to 60-90 minutes a day, 5 days a week) reduced their risk by 37%
* And finally, those fitness fanatics who exercised for 450-750 minutes a week (equivalent to 90 to 150 minutes a day, 5 days a week) reduced their mortality risk by 39% 
The two big take-aways here?
First, even a modest exercise habit of 30 minutes a day, 5 days a week has a notable effect on reducing risk of early death.
And second, the exercise “sweet spot”, according to this research at least, seems to be 300-450 minutes a week, which works out to 60-90 minutes a day, 5 days a week (the law of diminishing returns starts kicking in when the exercise habit reaches 450-750 minutes a week).
Another study, this one from 2012, looked at self-reported physical activity and BMI data from more than 650,000 adults ages 21-90. The study authors found that exercising 75-450 minutes a week (equivalent to 15-90 minutes a day, 5 days a week) effectively increased life expectancy anywhere from 1.8 to 4.5 years.
Longevity was further extended by 7.2 years when an exercise habit (in this study, 150-299 minutes a week, or 30-60 minutes a day, 5 days a week) was combined with maintaining a healthy weight (healthy weight here being determined by BMI, which granted, has some well-known limitations as an accurate body composition metric). 
The bottom line? Condition-appropriate exercise for a total of 30-90 minutes a day, 5 days a week, will help you optimize your lifespan and avoid an early death.
What do I mean by "condition-appropriate exercise"? Simply put, exercise that is appropriate for the unique physical condition of the individual. Risk of injury tends to increase as the movement progresses from...
* simple to complex
* low intensity to high intensity
* low load (i.e. weight) to high load
* short duration to long duration
* low reps to high reps
* no/low impact to high impact
For example, "condition-appropriate exercise" for a relatively deconditioned middle aged or elderly individual with one or more joint restrictions, respiratory and/or cardiovascular risk factors and a significant amount of excess weight is going to look very different than condition-appropriate exercise for a relatively well-conditioned person in their 20s or 30s with no major joint restrictions.
TIP FOR OVER-ACHIEVERS:
TRY TWO-A-DAYS (WITH SHORTER DURATION)
If you REALLY want to optimize your health and fitness efforts, you might try splitting your daily workout time up into two sessions. In other words- try a “two-a-day.” While it’s an added logistical challenge to be sure, I’ve seen individuals get better results doing two exercise sessions a day. This might look like a 30 minute walk/jog with self-massage and stretching in morning, followed by a 30 minute strength-based workout in the late afternoon, versus one 60 minute workout in the late afternoon or early evening.
Besides increasing exercise frequency (which has benefits for the musculoskeletal, respiratory, circulatory and lymphatic systems), the “two-a-day” habit also works to decrease exercise duration, which can help to reduce the body’s exposure to potentially harmful metabolic by-products (including free radicals) that can come with moderate to high intensity, longer-duration exercise.
And since the average American sits anywhere from 6.5 to 10 hours a day (depending on the research looked at), it probably doesn’t hurt to infuse that sedentary lifestyle with two intentional physical activity sessions instead of one. 
"It is not God, but people themselves who shorten their lives by not keeping physically fit."
~Carl Linnaeus, 18th Century Swedish Botanist, Zoologist & Physician
"For physical training is of some value, but godliness (spiritual training) is of value in everything & in every way, since it holds promise for the present life & for the life to come."
~1 Timothy 4:8 (Amplified)
2 www.ncbi.nlm.nih.gov/pmc/articles/PMC1402378/ (2006; review of health benefits of exercise)
3 www.ncbi.nlm.nih.gov/pmc/articles/PMC2885312/ (2010; review of health benefits of activity & exercise in school aged youths)
4 www.ncbi.nlm.nih.gov/pubmed/27017149 (2016; vigorously intense exercise increases risk of adverse CV events)
5 www.ncbi.nlm.nih.gov/pmc/articles/PMC3538475/ (2012; potential adverse CV events from excessive endurance exercise)
6 www.ncbi.nlm.nih.gov/pubmed/15534432 (2004; in general, benefits of exercise outweigh the risks)
7 www.ncbi.nlm.nih.gov/pmc/articles/PMC4451435/ (2015 review of data of more than 660,000 people to ascertain how exercise quantity reduces all cause mortality)
8 https://journals.plos.org/plosmedicine/article?id=10.1371/journal.pmed.1001335 (2012 review of data from >650k people found that exercise & maintaining a healthy body weight increased life expectancy after 40 by 1.8 to 7.2 years)
9 https://jamanetwork.com/journals/jama/article-abstract/2731178 (2019; reports of survey data from 2001-2016 finds American adults are sitting 6.5 hrs/day and children 8.2 hrs/day)