How fit do we really need to be?

If 2025 proved one thing, it’s that Americans’ interest in maximal fitness is higher than ever. 

Participation in running races broke records. The New York City Marathon in November had almost 60,000 finishers, breaking the previous year’s record by several thousand, and ultrarunning events, which involve running longer-than-marathon distances, recorded historic participation levels. Meanwhile, extreme workout regimens are going viral. That includes 75 Hard, which dictates 75 consecutive days of drinking a gallon of water, forgoing alcohol, following a new diet of your choice, reading 10 pages of nonfiction, and twice-daily 45-minute workouts (one of which must be outside).

While there is also undoubtedly a social component to this exercise maximalism, the aim of all this exertion is to be “fitter,” a word we tend to use as a synonym for “healthier.” We all know that getting regular exercise is critically important for our physical and mental well-being, but what’s less clear is just how fit we need to be for health. Should we all be striving to be ultra-fast marathoners, or is there a middle ground in fitness, where we can be just as healthy without those lofty exercise goals? After all, far from all of us will become elite athletes. When it comes to fitness, is more really more?

The answer is a little complicated. Improving your fitness through exercise affects health in a number of ways. It reduces the risk of diseases like heart failure or dementia, but it also makes your body more functional, so you’re able to open heavy doors or carry your groceries. Researchers have found that the biggest improvement to health — for longevity but also your quality of life — appears when a non-exerciser begins to add some light exercise in their routine. Going from only light exercise to more intense or prolonged exercise does have additional health benefits, but there do seem to be diminishing health returns on the investments of your time and efforts. Complicating the issue is the fact that fitness can’t be boiled down to just one thing, or one measure.

What exactly is fitness? 

Experts typically divide fitness into two categories you are probably familiar with: muscular strength and cardiorespiratory (or cardiovascular) fitness. 

Muscular strength is simply how strong you are, and can be developed by resistance training like lifting weights. The primary connection between muscular strength and health is in the fact that maintaining good muscle mass prevents frailty and allows people to do the activities they want to do without injury. 

The benefits that resistance training in young adulthood might provide for long-term health and longevity are a little unclear. But what is clear is that maintaining muscular strength is incredibly important for the health of older adults, since muscles weaken as you age. There’s also some mixed scientific evidence suggesting that resistance training can possibly improve brain health, help with managing Type 2 diabetes, and reduce cancer mortality.  

Cardiorespiratory fitness is how well your body can utilize oxygen while exercising, and is improved via aerobic exercises, including running or biking. There’s much more robust research connecting cardiorespiratory fitness to health than with strength fitness, with evidence showing that it reduces the risk of heart disease and death by cancer, and it also improves longevity in general.

This type of fitness is typically measured using a metric called VO2 max, which describes how many milliliters of oxygen the body consumes per kilogram of body weight per minute of exercise. To complete any function, our bodies require a constant supply of oxygen, which is supplied as our lungs breathe in air, hearts pump oxygenated blood throughout the body, and muscles extract and make use of that oxygen. If any one of these components is weak or inefficient, that limits the amount of exercise the body can complete as it struggles to meet oxygen demands. So a higher VO2 max means a person’s body is better at transporting and utilizing oxygen, which typically means they’re capable of more exercise.

Another important measure of cardiorespiratory fitness is metabolic equivalents (METs), which is the ratio of how hard your body is working during any task or exercise compared to when your body is at rest. So a MET of 3 means your body is working 3 times harder than if you were sitting still. Improving cardiorespiratory fitness will eventually allow you to exercise at higher METs than before and improve your VO2 max.

Together, strength and cardio make up what we think of as “fitness,” which is ultimately the ability to go about your daily life and all its activities without injury or discomfort. There is some research showing that people get the greatest health benefits when they exercise both components. There is currently much more granular evidence showing exactly how cardio benefits health and longevity compared to strength training. 

Is there a cap to fitness benefits?

When it comes to cardiorespiratory fitness specifically, researchers have found a clear link to longevity. “Individuals who have higher levels of [cardiovascular] fitness live longer,” says Mark Haykowsky, a professor at the University of Alberta who researches aging and quality of life. In a paper he coauthored, published in 2024, data revealed that people who can run a mile in under 4 minutes — an incredibly difficult feat that would require a high VO2 max — live on average about 5 years longer than their predicted life expectancy.

But you don’t need to be an elite runner to see longevity benefits from cardio. In a study of Danish men, researchers found that when participants had higher than average VO2 Max levels in middle age, they ended up living longer on average. A 2009 study from Japan found that just improving your maximum exercise abilities by 1 MET can decrease your risk of mortality by 13 percent.

In another study, Duck-chul Lee, a professor of health and human development at the University of Pittsburgh, found that even small amounts of running can affect fitness and longevity. The results suggested that running for just one hour can add seven hours to your life, regardless of your pace. “That one-to-seven ratio is really sweet,” Lee says. Of course, you cannot run away from death indefinitely, and by his numbers this benefit caps out at around 3 years (which you could hypothetically achieve by running an hour a day for more than 10 years). 

When it comes to muscular strength, however, the relationship between being stronger and living longer are less clear. “Interestingly, evidence would suggest that elite power and strength athletes don’t receive that same longevity benefit” as endurance athletes, Haykowsky says. It still could be possible that an hour of strength training has some effect on longevity, like an hour of running does, but “we need more data,” Lee says.

Regardless, immense physical strength by itself does not seem to be associated with better health. Bodybuilders, for example, tend to live shorter lives than the average population — though that link is muddied by the potential health impacts of steroid use.

So should we all be running marathons?

When it comes to cardiorespiratory fitness, it does seem that more is more. There’s no cutoff point where health benefits drop off. 

However, it’s important to distinguish fitness from exercise. While being fitter is better for health, that does not mean that more exercise is always better, says Lee. Doing intense vigorous exercise for prolonged periods won’t necessarily improve your fitness at a better rate than taking a more moderate approach. 

In his study, Lee found that “runners running less than one hour a week and runners running three hours or more per week showed almost equal benefits on mortality.” Running more than four hours didn’t lead to a decline in health benefits, simply a plateau. 

James Smoliga, a professor of rehabilitation sciences at Tufts University, is personally fed up with the popularity of marathons and wishes that people would focus more on doing higher-intensity exercise for shorter periods of time. Alternating between short bouts of intense cardio and rest, also known as high-intensity interval training, has been shown to improve VO2 max more effectively than doing moderate cardio for longer. “Being able to run a slow marathon doesn’t necessarily make you more fit than somebody that runs a good 5K,” he says. 

But how good is good enough? National guidelines recommend 150 minutes of “moderate” intensity aerobic exercise in a week — Smoliga says you can gauge “moderate” as the highest intensity exercise you can do while still carrying on a conversation. Over time, the bar for “moderate” will get higher. At first, you might only be able to chat while running at a 14-minute mile pace, but over the months or years, you might find you can still converse while pacing at a 10-minute mile. You’ll be able to do more while not necessarily feeling like you’re working harder, up to a point. When eventually that drop-off does happen, it’s fine to stay in that moderate-intensity band of workout, says Smoliga. You’re still moving your body and contributing to your health.

While you definitely don’t need to pay attention to measures like VO2 max or METs to work out well and improve your cardio over time, it can be a good way to track your own personal progress. Many fitness trackers these days will display estimates of your VO2 max, and some might even provide longevity scores based on that and other metrics. But while it can be encouraging to track how your VO2 max changes over time, especially since that’s the best measure of cardiorespiratory fitness, other simpler measures like pace or mileage can be just as good for gauging your progress. Plus, focusing too much on improving any one measure can distract from the one variable that should matter most when you’re exercising: Does your body feel better?

It’s also important not to discount the importance of strength training, especially in older age. Fitness, especially muscular fitness, naturally decreases throughout the lifespan, so staying strong past midlife becomes much more of an uphill battle. Preventing that loss of strength via strength training is important — weaker grip strength in older adults is linked to disability, heart disease, and other chronic diseases.

If you’re young, generally healthy, and able, it makes sense to pick cardio as your primary focus for exercise and fitness, says Haykowsky. There’s more evidence that you’ll get more long-term bang for your buck. But starting in your 50s, as your natural muscle tone declines, “I would say lift weights as your primary focus, but be sure to still be walking and moving as your secondary focus,” Haykowsky says. You still want to include some cardio, but the greater immediate threat to health will be frailty, so strength training should be the priority.

Things like balance and flexibility are also elements of fitness, says Smoliga, just less talked about. But they contribute to health in that they help you live your life without incident, so you don’t injure yourself while lifting a package or stepping out of the shower.

While there’s no one level of fitness we should all be striving for, experts say that the most important thing is that you’re exercising at all. Lee emphasized the fact that the greatest health bump experts observe in people happen when they transition from no fitness to some fitness. Going from some fitness to more fitness is good too, but getting started is what’s most important.