Exercise is good for you: there is a big difference in likely long term health between moderate regular exercise and being sedentary. Exercise seems to be roughly on a par with calorie restriction when it comes to improving health and extending average life span, but interestingly it doesn't extend maximum life span in the way that calorie restriction does in animal studies. There is that intriguing disconnect between improved long-term health and maximum observed longevity that someone, one day, will be able to explain: from a naive perspective that considers aging to be accumulated damage, you'd expect it that any improvement in health over the long term would tend to push out maximum life span.
While the difference between no exercise and moderate exercise (the traditionally recommended 30 minutes of aerobic exercise a day that every doctor will tell you about) is well supported by the evidence, it's harder to say that more is better, or that some exercise is better than other exercise. Human data shows us that athletes are longer lived than the rest of us on average, for example, but it's far from clear that they are long-lived because they exercise, versus there being a bias towards athletics as a career for more robust individuals who would have lived longer anyway.
Going by the published research, the 80/20 win for personal health involves taking the 30 minutes a day at this point. A recent paper suggests that it doesn't matter how you rack up the time so long as it's somewhat regular:
[Researchers] studied 2,324 adults from across Canada to determine whether the frequency of physical activity throughout the week is associated with risk factors for diabetes, heart disease and stroke. "The findings indicate that it does not matter how adults choose to accumulate their 150 weekly minutes of physical activity. For instance, someone who did not perform any physical activity on Monday to Friday but was active for 150 minutes over the weekend would obtain the same health benefits from their activity as someone who accumulated 150 minutes of activity over the week by doing 20-25 minutes of activity on a daily basis."
So in general keep in mind that the outcomes with exercise are much better than those without it. Living a sedentary life is a matter of stabbing yourself in the back thirty years down the line: making your future more expensive, more painful, and shorter. Perhaps the pace of medical science will keep up with you and you'll be rescued by new medical technologies - but why take risks that you don't have to?
There is structural and functional deterioration of almost all physiological systems during aging, even in the absence of discernible disease, resulting in reduced independence and increased incidence and progression of chronic diseases in older adults.
However, regular participation in physical activity and/or exercise training programs can minimize the physiological alterations that occur during aging and may contribute to improvements in health and well-being. Numerous studies have shown that exercise training programs improve the muscle strength, balance, cardiorespiratory fitness, metabolism, glucose tolerance, daily living activities and psychological health of elderly people, even those in their 80s or 90s. Accordingly, national and international agencies have recommended regular physical activity or exercise participation to promote older adult health and disease prevention.
In this context, avoiding a sedentary life style by performing any type or level of daily exercise is a prudent recommendation to follow as it will reduce the impact of aging on some physiological functions, reduce the risk of developing chronic disease and prevent premature mortality, regardless of age.
Ultimately, of course, you can't treadmill your way out of aging to death. What you can do is make life more likely to be pleasant and longer by a handful of years. The studies that compare those who exercise with those who don't suggest that the value of being active is 5 to 10 years of life expectancy, a bonus above and beyond the health benefits. If you want to live longer than that, and with greater certainty of a long future ahead, then the development of new medical technology is the only viable way forward. The earlier you start helping to make foreseeable technologies of human rejuvenation a reality, the more likely it is that they will exist in time to save you from the frailty, suffering, and death caused by degenerative aging.