It’s easy to double your risk of developing a serious health problem. Just don’t exercise.

Roughly 70% of American adults don’t meet current physical activity recommendations — that is, they have sedentary jobs (or sit most of the day)… don’t have a regular regimen of physical activity… and don’t do much work around their homes and yards.

Eye-opening finding: A low level of fitness is more dangerous than obesity, smoking, high cholesterol or diabetes, according to a landmark study.

It’s true that even a relatively small amount of exercise — done on a regular basis — can improve your health. Even moderately fit men live up to six years longer than unfit men, studies show.

But why not get the maximum health benefits — including an up to 50% lower risk for heart attack and stroke — each time you lace up your sneakers? Here’s how…

SECRET 1: Exercise at the right intensity. To protect your cardiovascular system (including the heart and blood vessels), you must perform aerobic activity. This type of exercise lowers your heart rate and blood pressure during times when you’re not exercising (reducing the workload of your heart and blood vessels)… and helps decrease your blood’s tendency to clot.

Aerobic activity includes many different activities, such as walking, jogging, swimming, cycling or working out on a stair-climber, elliptical machine, exercise bike or rowing machine. The key is to exercise at a sufficient intensity level.

So how do you know if you’re reaching that target intensity level? Scientists measure physical effort in METs — short for “metabolic equivalents.” One MET is the energy your body uses at rest. For improvements in heart-lung fitness, you need to exercise at 40% to 60% of your maximum MET capacity.

Example: If your maximum exercise capacity is 10 METs (meaning that when you’re exercising as hard as you possibly can, you expend 10 times as much energy as when at rest — for example, sitting on a couch), then your exercise intensity should be at least in the 4-to-6 MET range. (Many types of exercise machines provide estimated METs during exercise.)

The effort that is needed to reach a cardioprotective level of energy expenditure varies from person to person. For example, for an 80-year-old, walking 2.5 mph may be enough, while a younger person may need to walk briskly or jog.

Smart idea: If your doctor recommends that you get a treadmill stress test (a clinical test that involves walking on a treadmill while the incline is progressively increased to measure cardiovascular fitness), ask him/her for your maximum MET capacity. Your speed, incline on the treadmill and time achievements are factored into this number, but doctors often don’t mention it when reviewing stress test results with their patients. In general, the longer you last on the treadmill, the higher your MET capacity.

Another way to tell if you’re exercising at the right intensity: Use the Borg Scale of Perceived Exertion, created by Swedish exercise physiologist Gunnar Borg…

What you need to remember: If you stay in the 11-to-13 range during your workouts, you’ll be exercising at an intensity that is sufficient to improve cardiovascular health. In this range, you’ll be breathing harder, but you can still carry on a conversation. Less than that, and you’re not exercising hard enough to get optimal health and fitness benefits.

Important: One guideline that is widely used sets your target exertion rate at 70% to 85% of your estimated maximum heart rate (based on the formula 220 minus your age).

I strongly advise against using this approach because your maximum heart rate will vary widely based on your overall health, medications you’re taking and other factors, such as underlying heart disease or diabetes.

SECRET 2: Exercise for the right amount of time. Studies have shown that cardioprotective aerobic activities may be accumulated to trigger the physiological changes associated with improved cardiovascular health.

The good news is that exercising in several 10- to 15-minute bouts provides the same cumulative health benefits as longer workouts. So there is an alternative to the old, widely offered advice that you should exercise for at least 30 minutes per session — you can become just as healthy by doing three 10-minute workouts or two 15-minute workouts.

This approach allows you to “disguise your exercise” by incorporating multiple shorter workouts into your daily routine. Look for times during the day when you can fit in a brisk 10-minute walk and perhaps a 20-minute bike ride later in the day.

Ideally, you should take 8,000 to 10,000 steps daily. If you’ve been walking far less than that total, aim to increase your steps by 500 to 1,000 per day each week until you reach this level.

Smart idea: Get a reliable pedometer (a good one typically costs at least $20) to count the total number of steps you take each day.

Good brands: Yamax Digiwalker and Omron Pocket Pedometer. (Both are available online.)

In a review of 26 randomized controlled studies, researchers found that participants in exercise programs who wore pedometers walked about a mile and a quarter more per day, on average, than exercisers who didn’t wear them.

SECRET 3: Start exercising gradually — but start! When I’m working with people who haven’t exercised in quite a while, I encourage them to start by exercising for 10 to 15 minutes at a time, three to four times per week. That may not sound like much — but if you go from doing nothing to doing this amount of exercise, you’ll improve your health more than a person who goes from running 20 miles per week to running 25 miles a week.

How could this be? Recent pioneering research has shown that people who don’t exercise at all have the greatest risk by far for cardiovascular illness. If you get out of this inactive category by doing even a modest amount of cardioprotective exercise each week, you’ll substantially decrease your risk for a heart attack or stroke.

As a long-term goal: Build up to at least 150 minutes of exercise at a cardioprotective intensity (11 on the Borg Scale) or 75 minutes of somewhat harder exercise (13 on the Borg Scale) per week — or an equivalent combination of exercise performed at a cardioprotective intensity.

This is the amount of exercise recommended in the latest physical activity guidelines by the US Department of Health and Human Services. The exercise can be divided into shorter sessions of at least 10 minutes spread out over the course of each week.

As you’re gradually building up to this goal, don’t be discouraged if you have trouble reaching it.

Remember: Any cardioprotective exercise is much, much better than none at all!