Dr. Bert Mandelbaum for U.S. News & World Report: The Key to a Longer Life: Exercise More

ABOUT 50 YEARS AGO, THE average life expectancy for men in the United States was around 67 years, and for women, it was approximately 74. Today, American men live to about 77 years old on average; women, to about 81. There's no question that the average life expectancy has increased over the last several decades. Men are living approximately 10 years longer and women about seven years longer. But how can we maintain this positive trend, or even better, increase both the quantity and quality of those years?

A study that was recently published in The American Journal of Epidemiology aimed to answer that question with somewhat of a simple formula that just about everyone can work with: Replace 30 minutes of the time you spend sitting every day with 30 minutes of light exercise. The payoff, according to the study? About a 17 percent lower risk of dying early. And if a reduced risk of premature death wasn't enough, the study found even more good news. Increasing that exercise intensity from light to oderate or vigorous intensity lowered the risk of early death to 35 percent. The bottom line is this: Moving more does a body good.

Indeed, exercise and nutrition are the elixirs of a healthy life that's full of quality in the number of years lived. I know that favorite "gimmicks" would have you believe there's a magic bullet to health that allows you to forgo consistent exercise and healthy foods. But those magic bullets are traps, and deep down, everyone knows it. But why is doing what we know we need to do regarding our diet and fitness so incredibly hard? Well, there's a reason that idiom "old habits die hard" exists. Trying to break free from a cycle of sedentary behavior or unhealthy eating – especially if you've engaged in it for the majority of your years so far – isn't just hard; it's downright daunting. So what can you do to make it less so, and to make sure you're not approaching this one life you've got sitting down?

Start small. Habits take time to form and time to break. And the range between how long a habit takes to build in one person can vary wildly from how long it takes someone else. It's a marathon, not a sprint, and we've already established that there's no shortcut. So starting with bite-sized goals is an excellent plan. If getting 30 minutes of exercise every day isn't something you think you can fit in just yet, try breaking it into smaller sections. Park further away from a location and take more time to walk there, take the stairs instead of the elevator at school or work and get up and perform a chore if you've been seated on the couch for more than a half-hour. All of these efforts could add up to those 30 minutes you need each day, without you even realizing it. But do track them and hold yourself accountable to them.

And of course, I would be remiss if I didn't reiterate what you've probably heard more than once: You can't out-exercise a poor diet. It's true. You can't. No matter how lofty you make those fitness goals, if your diet consists mainly of heavily processed, fast foods, you'll be sabotaging all the efforts you're making on the exercise front. Again, keeping the "start small" mantra in mind, begin to look at what and how you eat and commence making little changes here and there. Swapping processed potato chips at lunch for an apple or other whole fruit, replacing one heavily sweetened beverage with water each day, trying one new "healthier" food option each week – you get the point. People don't want to fail when they set out to get healthy. And once we've perceived failure, psychologically, we're also more likely to give up and slide back into what's comfortable, even when we know it's unhealthy. Don't set yourself up for that.

No one wants to feel burdened by a body that won't perform at a level that's worthy of quality living. Our ticket to avoiding such a fate is through an intentional and consistent focus on exercise and nutrition. Slow and steady, small changes leading to big ones. I've often told my patients that the habits we develop in the best of times will be available to us in the worst. When we build regular activity and the right nutrition into our lives, we're creating physical conditions that help us excel in the sport of life – for the majority of the years we have left.