I have always been pretty much a failure at anything athletic. I typically was among the last chosen for any sports team. I didn’t learn to ride a bike until I was 11. In college I was required to take four semesters of physical education; I tried skiing and bowling and would have flunked both had I not aced the written exams. I quit tennis after 10 lessons because I couldn’t master eye-hand coordination.

Not until the fitness boom of the ‘70s did I find my niche when I discovered I had an aptitude for exercise, perhaps because what I lack in athletic skill I make up for with a natural sense of rhythm and flexibility. Jane Fonda became my hero. I did it all — aerobics, free weights, jogging, walking, stretching, Zumba, Pilates — and, except for jogging, I’m still at it.

For the last 40 years I have regularly worked out 40 to 60 minutes, five to six days a week. When I was diagnosed with stage two breast cancer in my early 60s, I sailed through radiation because I power walked back and forth from Rittenhouse Square to Hahnemann Hospital every weekday throughout my months-long treatment. In my late 70s, I was diagnosed with a blood cancer called MDS (myelodysplastic syndrome). My doctors tell me that after three years of chemotherapy, it’s my fitness level that’s contributed to my remission and allowed me to maintain my energy and endurance.

Just as I have been depositing money in my IRA retirement account for decades, I’ve been steadily building reserves in my health account with a continuous, long-term exercise regimen.

Nearly all my friends who are 80-plus engage in some kind of exercise. Bud walks an hour or more a day; Sue does Pilates and rows on the river; Mary and Howard play tennis; Hope does barre classes; Eve and Lynn are on a dragon boat team. And then there’s my contemporary Marian: She bikes, walks, does yoga, takes salsa and hip-hop classes with women less than half her age. My active cohorts are living proof that the more you accumulate in your exercise bank, the more you will have to nourish you when your body has less to give. We have all made considerable investments to protect the quality of our lives as octogenarians.

Over the last three decades, extensive research has indisputably established that physical inactivity is a cause of disease and disability, while sustained exercise bolsters physical durability, endurance, strength, immunity and mental prowess. It’s practically impossible to find an aspect of healthy aging that’s not improved by exercise: mood, sleep, heart function, muscle mass, and more. Fitness creates a barrier to conditions from diabetes to osteoporosis to dementia.

And while there is truth to the axiom that it’s never too late to get some benefit from exercise, those gains will never be matched by the benefits you accumulate from starting early and not stopping until the battery in your clock goes kaput. “You can’t outrun your genetics,” said Cedric Bryant, chief science officer at the American Council on Exercise. “Genetics load the gun, but lifestyle pulls the trigger. Exercise can delay when that trigger gets pulled. It keeps your biological age well below your chronological age.”

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There are many reasons why exercise is the closest thing to the fountain of youth, starting at the cellular level.

All our cells contain tiny caps called telomeres that are like aglets on shoelaces sitting at the end of each strand of DNA. As cells grow older, telomeres naturally shorten and fray, which contributes to cellular aging. Studies suggest that master athletes have longer telomeres than sedentary people of the same age. Moreover, a 2015 study published in the journal Medicine & Science in Sports and Exercise that enrolled 6,500 participants from ages 20 to 84 found that even sustained, moderate exercise may slow the fraying process. Researchers concluded that the more years people exercised, the less likelihood they had of shortened telomeres and a shorter life.

Another major study conducted in 2018 by the University of Birmingham and King’s College in London examined 125 amateur cyclists aged 55 to 79. The cyclists, who’d exercised throughout their lives, outranked the inactive control group in every benchmark. They had better preserved muscle mass, lower body fat and cholesterol, and immune systems that functioned like those of people decades younger. They had what academics call physiologic reserve.

Scott Trappe, director of the human performance lab at Ball State University, defines physiologic reserve as the level of oxygen available in the body’s fuel tank. Frailty occurs when your heart can’t distribute enough oxygen to service your physical needs. Exercise prevents your tank from draining to empty. Trappe’s lab examined a large group of people 70 and older who’d started “recreational fitness” during the exercise boom of the ‘70s and never stopped.

“By every measure of data, these [lifelong exercisers] looked like people 30 years younger,” Trappe said. “Their metabolic fitness was almost indistinguishable from young exercisers.”

The good news is that you don’t have to train like an Olympic contender to get the perks of long-term exercise. Fitness is measured in something called METS, which is a gauge of your metabolic rate based on the amount of energy you burn. On a scale of 1 to 10, the goal for a recreational exerciser is a mere 3-6 METS.

Matthew Silvis, a sports medicine specialist at Penn State Health, said just 30 minutes of moderate aerobic activity for five days a week can be enough to achieve that goal. He suggests things like a brisk walk at a conversational pace, basketball, biking at 10-12 mph, dancing, swimming, doubles tennis, or climbing the stairs in three 10-minute intervals a day. These will take care of your cardio-respiratory needs, but no single exercise will cover all your bases. You need to include strength training, stretching and balance exercises, as well.

“The earlier you get into the habit of diverse, ongoing, moderate exercise and the longer you keep at it, the more you will improve the quality of your aging and reduce the risks of problems that go along with it,” he said. For example, he points to someone who has a family history of heart disease. “Consistent exercise can lower that risk to the level of someone who has no family history at all.”

Diabetes is another condition that can be moderated by exercise. An estimated 34 million Americans have adult onset (type 2) diabetes, a disease that interferes with the metabolism of insulin.

“It’s been proven that exercise can regulate genes,” said Greg Ruegsegger, professor of exercise science at the University of Wisconsin. “Genes represent about half the causes of human mortality, but exercise and diet choices influence the other 50%.” In fact, a National Institutes of Health study showed that diet and exercise can reduce the risk of type 2 diabetes by 58%.

The most valuable gift you can give yourself is a commitment to exercises that strengthen your heart so it doesn’t weaken and atrophy. The more you demand of your heart muscle, the more efficiently it pushes out blood supply, the more oxygen it delivers to your organs and the more endurance you develop. “Exercise capacity [known as VO2 max] increases the maximum amount of physical exertion your heart can tolerate,” Ruegsegger said. In one study, it was a “more powerful predictor of mortality among men than any other risk factor for cardiovascular disease.”

It’s the same story with the musculoskeletal system. You can develop some muscle at any age, but most of your bone mass is acquired by the time you reach 20, and after that it levels off or declines, especially for women after menopause. Meanwhile your tendons and ligaments are losing elasticity, like an old rubber band that sits too long in a drawer.

“Workouts break down muscle at a micro-level, triggering the body to respond by building new muscle,” said Nicholas DiNubile, an orthopedic surgeon in Delaware County, sports medicine specialist, and the author of FrameWork: Your 7-Step Program for Healthy Muscles, Bones, and Joints. “The only way to increase bone strength and muscle mass is by loading your skeleton and its support systems — those ligaments and tendons — with the stresses of strength training. The more you load, the more you build back better.”

Last, but by no means least, ongoing exercise is what neuroscientist Wendy Suzuki called “a supercharged, 401(k) for your brain. It slows down those parts of the gray matter that are most susceptible to decline. It’s truly transformative.”

In her popular 2017 Ted Talk, she describes how exercise generates new cell growth in the hippocampus (the part of the brain related to memory), improves focus and attention, and produces those “good mood” neurotransmitters, such as dopamine and serotonin, that ward off depression and anxiety.

“Exercise is the only medicine for your entire body that you don’t need to swallow,” said Lyndon Joseph, an exercise physiologist with the NIH. “We don’t yet understand enough about the mechanisms of how exercise works, but the results speak for themselves. Exercise, when maintained for decades, is the most effective agent we have to fight the deterioration of aging.”

So I am shutting my computer and going for a long walk. My intention is to die young — as late as possible.

Carol Saline is a freelance health writer who lives in the Philadelphia area.