Photo: Ramin Talaie/Corbis

I ran 6 miles this morning, I’ll run 5 tomorrow, and on Saturday morning I’ll run 20. I’m running the New York City Marathon next month, and this training schedule, by the way, is about as chill as marathon training schedules ever get. Still, it’s a lot of miles. And so you would think all of this running must come with the happy side effect of some substantial weight loss, wouldn’t you?

I would, too, and yet this is not at all what has happened. I seem to have either stayed at exactly the same weight I was before I started this thing or, if anything, put on a few pounds. It’s the second marathon I’ve run, and this happened last time too. What kind of cruel physiological quirk is going on here? The answer, as it turns out, has implications that are more far-reaching than marathons, and hint at the fact that the way Americans currently think about exercise is not ultimately very accurate or helpful.

When Mary Kennedy coached a charity team of marathon runners back in 2009, she regularly heard the marathon-weight question. “Several of them would come to me and they would say exactly that: ’I am working out more than I ever have in my entire life. I’m doing this for a lot of reasons, but I really thought I’d look better in my clothes,’” said Kennedy, who is an exercise physiologist at the Institute of Lifestyle Medicine, a nonprofit research center founded in 2007 by Harvard Medical School and the Spaulding Rehabilitation Hospital. She conducted a small, simple pilot study, limited to her group of 64 charity runners, comparing their weight before starting the training program to their weight after completing it. About 11 percent of them did lose weight, but just as many gained weight (and of those who gained, 86 percent were women). But for the remaining 78 percent, their weight stayed almost exactly the same, even after three months of running four days a week.

Her results aren’t published yet, but they echo those of a 1989 study in which Danish researchers took 18 months to train a small group of sedentary people — 18 men and nine women — to run a marathon. By race day, the men had lost an average of five pounds. For the women, on the other hand, “no change in body composition was observed,” the researchers write. “This idea that you’re going to run a marathon and the pounds are going to melt away is not realistic,” Kennedy said. She’s currently coaching a group of high-school runners, and she and her co-director have a sad little joke: “You train for the marathon, and then you do the weight-loss program afterward.”

Sadly, the marathon weight gain isn’t likely a result of increased muscle mass, Kennedy said. Instead, the reason this happens has more to do with psychology than physiology, with much of it stemming from a misunderstanding of the purpose of exercise. For one, research suggests that most people assume they’re exercising harder than they actually are. Last year, a team of Canadian scientists published a study in which they asked their volunteers to run on a treadmill at what they felt was an easy pace and then pick it up to a vigorous one — with vigorous defined as increasing their heart rate to somewhere between 77 and 93 percent of its full capacity. The majority of the participants did just fine at estimating the easy pace, but not so for the vigorous one; most of them didn’t even increase their heart rate to 75 percent.

And this overestimation may be something novice marathoners are especially likely to do, most of whom are encouraged to adopt the “just finish” goal; it’s about getting the miles done and getting across the finish line, not running at any particular pace. That may mean they’re running a lot slower than they’re capable. “I don’t mean this in any derogatory way, but marathoning has really become accessible for everyone — which is amazing and wonderful,” Kennedy said. “But just because you cross the finish line doesn’t mean you were running at a really vigorous pace seven days a week … You’re so focused on going far, so you’re not necessarily doing a vigorous run.” Additionally, running a marathon sounds like a really impressive, intimidating goal — and, on the one hand, it is. But “if you really start to look at a lot of the novice training programs, people are running three or four days a week — it’s not an overly aggressive workout schedule. Even if you’re not training for a marathon, going to the gym three or four times a week — that’s not that out of the ordinary.” And this, by the way, is assuming that you actually do every single workout on your training plan.

Additionally, there is the small fact that exercising a lot makes you really, really hungry. “I’m ravenous after a long run,” Kennedy said. It’s incredibly easy to overeat without entirely realizing you’re doing it, and there also may be a misconception among novice runners about the importance of carb-loading. Carbs are important, sure, but it doesn’t mean “you have a giant plate of spaghetti with four pieces of bread,” she continued.

Beyond that, the entire idea that the purpose of exercise must be to lose weight — to pay a penance for the junk you’ve consumed — is not ultimately a very useful one. “It helps with mental health, it helps with so many things, but we focus so much on weight loss,” Kennedy said. “We make exercise overwhelmingly annoying. You need to do it, but you should find joy in it, because, my goodness, it’s doing so many good things for you.” In fact, thinking of physical activity in terms of weight loss tends to backfire, anyway. In one study published last year by Cornell University’s Brian Wansink, people who were told to take a two-kilometer “exercise walk” ended up consuming 124 percent more M&Ms afterward than those who were told they were taking a “scenic walk.” You’d be better off thinking of exercise as something to be enjoyed, in other words, rather than something to be suffered through.

One way to do that could be to keep in mind all the benefits that physical activity brings: It improves your mood, increases your energy, helps you sleep, and probably improves your sex life, too. For marathons in particular, the training often brings with it new running buddies, and relationships are among the most reliable predictors of well-being; the same goes for setting and making progress on a goal. Besides, “losing weight” is going to feel like a very flimsy motivator somewhere around mile 22, when you’re exhausted and bored and desperate for an excuse to stop running. There are many reasons to run a marathon, and there are many good reasons to exercise, but the link between physical activity and weight is probably not as clear-cut as many of us would like to think.