The questions lurk in the back of many of our minds—and here are the answers, according to two experts Fortune spoke to.

For adults 18-64 years old, the U.S. Center for Disease Control and Prevention recommends at least 150 minutes each week of moderate-intensity physical activity or 75-minutes each week of vigorous-intensity activity, along with two days of muscle strengthening.

Some further tailored recommendations from the CDC:

• Adults ages 65 and older should add to this routine activities to improve balance, such as standing on one foot. 
• Adults with chronic health conditions and disabilities should get 150 minutes a week of moderate-intensity activity, with two days a week of muscle-strengthening work. 
• Pregnant and postpartum women should get at least 150 minutes a week of moderate-intensity activity, with no muscle-strengthening or balance work.

There is solid, evidenced-based research to back up those numbers, Drew Contreras, a physical therapist and vice president of clinical integration and innovation at the American Physical Therapy Association, tells Fortune.

The recommended length of time and intensity is what’s needed to benefit your heart, he says. But shorter stints can still improve your health. Some of the potential positive effects, according to the CDC:

• reduces your chances of developing diabetes and metabolic syndrome
• helps your body handle illness better
• reduces your risk of some cancers
• strengthens your bones and muscles
• increases your chances of living longer

Moderate versus vigorous activity

Just what’s the difference between moderate- and vigorous-intensity activity? Moderate-intensity activity entails “working hard enough to raise your heart rate and break a sweat,” according to the CDC. If you’re doing this level of exercise, you should be able to talk but not sing a song. Some examples include:

• doing water aerobics
• riding a bike on level ground or with few hills
• playing doubles tennis
• pushing a lawn mower

Vigorous-intensity activity entails breathing “hard and fast,” and pushing yourself to the point that “your heart rate has gone up quite a bit,” the CDC states. If you’re doing this level of activity, you won’t be able to say more than a few words before needing to breathe. Some examples include:

• jogging or running
• swimming laps
• riding a bike fast or on hills
• playing singles tennis
• playing basketball

You can do an equivalent mix of moderate and vigorous activity, according to the CDC. And if you’re curious what qualifies as muscle strengthening, it’s a workout of all major muscle groups, including legs, hips, back, abdomen, chest, shoulders, and arms.

Does walking count?

If you’re walking to help meet your weekly exercise goal, are you wasting your time?

Not at all, according to Contreras. 

“If you’re walking at a pace where you’re a little out of breath but not yet running, and it’s a little harder to have a conversation, I would classify that as a moderate-level” exercise, he says.

Don’t be discouraged if your walking is more leisurely, Cedric Bryant, president and chief science officer of the nonprofit American Council on Exercise, tells Fortune. The activity “is oftentimes a person’s entry point into developing a regular, more robust” exercise routine. 

It’s “so accessible for everyone,” he says. “It doesn’t require any special instruction, and you don’t have to invest in a lot of expensive equipment.”

He encourages leisurely strollers wanting to step up their game to try “speed play,” which involves walking more briskly for short intervals, perhaps from one stop sign to the next. Walk at a leisurely pace for five to 10 minutes and then repeat, he recommends.

“That will serve a lot of people well,” he says.

Something is better than nothing

Both Contreras and Bryant encourage would-be exercisers not to be intimidated by an ideal number of minutes per week.

For a person who is “truly sitting on the couch and doing nothing, even less activity than [the recommended amount] can produce some pretty significant benefits,” Bryant tells Fortune.

“Basically, the lower you are on the fitness scale, the smaller the dose can be and still elicit a positive response. If we can get those people to do five to 10 minutes daily as a starting point, they’re going to benefit greatly.”

Adds Bryant: “As the old adage goes, some exercise is better than none, but more is better than some.”