They say a journey of a thousand miles starts with a single step. If the thought of a thousand-mile journey makes you tired, we've got some great news for you: You've probably already completed one.* A new study published in the journal Health Psychology [PDF] finds that people underestimate the amount of exercise they're getting—and that this underestimation could be harmful.

Psychologists at Stanford University pulled data on 61,141 American adults from two huge studies conducted in the 1990s and the early 2000s: the National Health Interview Survey and the National Health and Nutrition Examination Survey. Participants answered questionnaires about their lifestyles, health, and exercise habits, and some wore accelerometers to track their movement. Everybody was asked one key question: "Would you say that you are physically more active, less active, or about as active as other persons your age?"

The researchers then tapped into the National Death Index through 2011 to find out which of the participants were still alive 10 to 20 years later.

Combining these three studies yielded two interesting facts. First, that many participants believed themselves to be less active than they actually were. Second, and more surprisingly, they found that people who rated themselves as "less active" were more likely to die—even when their actual activity rates told a different story. The reverse was also true: People who overestimated their exercise had lower mortality rates.

There are many reasons this could be the case. Depression and other mental illnesses can certainly influence both our self-perception and our overall health. The researchers attempted to control for this variable by checking participants' stress levels and asking if they'd seen a mental health professional in the last year. But not everybody who needs help can get it, and many people could have slipped through the cracks.

Paper authors Octavia Zahrt and Alia Crum have a different hypothesis. They say our beliefs about exercise could actually affect our risk of death. "Placebo effects are very robust in medicine," Crum said in a statement. "It is only logical to expect that they would play a role in shaping the benefits of behavioral health as well."

The data suggest that our ideas about exercise and exercise itself are two very different things. If all your friends are marathoners and mountain climbers, you might feel like a sloth—even if you regularly spend your lunch hour in yoga class.

Crum and Zahrt say we could all benefit from relaxing our definition of "exercise."

"Many people think that the only healthy physical activity is vigorous exercise in a gym or on a track," Zahrt told Mental Floss in an email. "They underestimate the importance of just walking to the store, taking the stairs, cleaning the house, or carrying the kids."
*The average American takes about 5000 steps per day, or roughly 2.5 miles. At that pace, it would take just a little over a year to walk 1000 miles.