Two people work on the same London double-decker bus. One drives the bus. The other walks around collecting passengers’ fares all day. Who is more likely to die of heart disease?

We have known the answer for sixty years. The renowned epidemiologist Jerry Noah Morris researched this question in 1953 and, not surprisingly, found that the sedentary drivers were significantly more susceptible to heart disease than were the ambulatory conductors. The same bus, the same hours—let’s even for now assume the same salary—but not the same rate of heart disease. And so, if you were the driver and were offered the conductor’s job, would you accept that substitution?

I believe that the simple substitutions we unthinkingly make, day in and day out, determine our fates. And that is why I stand up in class. I have been standing for almost two years now—two years of exercising my soleus muscles instead of letting them dangle. Now, a common excuse for not exercising, and one for which I have been guilty, is a lack of time. But I know that standing up in class takes exactly as much time as sitting, yet look at the long-term benefits! I have no bloodwork data for myself, but innumerable research studies have found that standing more and sitting less is associated with lower insulin spikes, lower cholesterol and triglycerides, and lower risk of death. All by one simple substitution.

Now, take standing on the job a step further—or maybe ten thousand steps—and voila, the treadmill desk. While the benefits of standing are largely invisible, the benefits of walking are countable data—by mile or by step. While writing his fourth book, Drop Dead Healthy, A. J. Jacobs walked twelve hundred miles on his treadmill-laptop desk, substituting it for the sitting desk where he wrote his first book, The Know-it-All, when he probably just walked to the bathroom.

I too made walking habitual towards the end of tenth grade, walking to school instead of riding the bus. To walk one way took twenty minutes, and to ride took ten, but I regarded those twenty minutes as wholly productive, and those ten as sedentary waste. On days I walked both ways, my pedometer clicked five thousand extra steps. And so, by taking only twenty extra minutes each day, from the end of tenth grade until the end of twelfth, I substituted nearly eight hundred walking miles for eight hundred folded in a cramped, yellow bus.

I believe that many maladies that exist today—heart disease, obesity, and such—result in part from substituting comfort or convenience for salubrity. But I also believe that by pushing ourselves outside our comfortable niches—by substituting back the healthy behaviors we were meant to do, we can reverse these epidemics. Will that be comfortable? The first two weeks I stood in school, explaining what in the heck I was doing embarrassed me. My calves and ankles grew stiff and sore. But the pain did subside. And there were many days—frigid, blustery days—when I yearned to be seated in a heated bus as I pushed against the wind. But I kept walking.