THE BENEFITS (AND RISKS) OF EXERCISE
Exercise, when performed smartly, carries with it several benefits, including…
*chronic pain reduction
*improved cardiac & respiratory functioning
*reduced risk of cardiovascular disease & hypertension
*reduced risk of diabetes
*reduced risk of cancer
*increased amounts of lean muscle
*reduced body fat
*reduced risk of obesity
*improved bone density
*reduced risk of osteoporosis
*reduced risk of depression
*improved sex drive/libido
*an excuse to get out in nature (which carries its own benefits- fresh air, sunshine, vitamin D, etc.)
*positive stimulation of various body systems (lymphatic, circulatory, digestive) [1-2]
Of course exercise can have adverse effects, especially when dangerous and/or inappropriate activities are routinely applied. Risks here include…
*increased risk of injury
*trouble breathing (which can be dangerous for asthmatics & 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 metabolic by-products (which can damage cells and tissues over time)
*rhabdomyolysis (breakdown of muscle tissue)
*cardiovascular risks (including heart attack or stroke) [3-4]
Despite these risks research shows that a certain amount of exercise is generally associated with a reduced risk for early death (i.e. decreased mortality rates). 
HOW MUCH EXERCISE DO WE NEED TO AVOID AN EARLY DEATH?
So how much exercise is ideal for reducing your chances of an early death?
A very large 2015 JAMA study that looked at data from more than 660,000 people 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 exercise-lovers who exercised for 450-750 minutes a week (equivalent to 1.5-2.5 hours a day, 5 days a week) reduced their mortality risk by 39% (only slightly better than the 300-450 min/week group, a good example of the law of diminishing returns) 
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.
Second, the exercise “sweet spot” according to this research seems to be 300-450 minutes a week, which works out to 60-90 minutes a day, 5 days a week.
A 2012 study that looked at self-reported physical activity and BMI data from more than 650,000 adults ages 21-90 found that exercising 75-450 hours a week effectively increased life expectancy anywhere from 1.8 to 4.5 years. Longevity was further increased when a physical activity 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 has some well-known limitations as an accurate body composition metric). 
The bottom line? Appropriate exercise for a total of 60-90 minutes a day, 5 days a week will help you optimize your lifespan and avoid an early death.
Image Source: http://healthymidlife.com/exercise-can-extend-our-lifespan.html
TIP FOR OVER-ACHIEVERS: TRY TWO-A-DAYS WITH SHORTER SESSIONS
If you REALLY want to optimize your health and fitness efforts, 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 (ex. a 30 minute workout in morning and a 30 minute workout in the late afternoon) versus one session a day (ex. a 60 minute workout in the evening).
Besides increasing exercise frequency (which has benefits for the musculoskeletal, respiratory, circulatory and lymphatic systems), the “two-a-day” habit also decreases exercise duration, which can help to reduce the body’s exposure to potentially harmful metabolic by-products that can come with moderate to high intensity, long-duration exercise.
And since the average American sits anywhere from 6.5-10 hours a day (depending on the research looked at), it probably doesn’t hurt to infuse that sedentary lifestyle with two intentional physical activity/exercise sessions instead of one. 
1 www.ncbi.nlm.nih.gov/pmc/articles/PMC1402378/ (2006; review of health benefits of exercise)
2 www.ncbi.nlm.nih.gov/pmc/articles/PMC2885312/ (2010; review of health benefits of activity & exercise in school aged youths)
3 www.ncbi.nlm.nih.gov/pubmed/27017149 (2016; vigorously intense exercise increases risk of adverse CV events)
4 www.ncbi.nlm.nih.gov/pmc/articles/PMC3538475/ (2012; potential adverse CV events from excessive endurance exercise)
5 www.ncbi.nlm.nih.gov/pubmed/15534432 (2004; in general, benefits of exercise outweigh the risks)
6 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)
7 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)
8 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)