Walking, upright on two legs, is one of the defining characteristics of humanity. We are evolved to walk – almost everything about your body, from the weight distribution of your upper and lower torso to the joints of your feet, ankles, knees, and hips, to the balance-providing structures of your inner ear, are fine-tuned by evolution to optimize the bio-mechanics of walking.

That evolution means that walking is one of the most natural ways to get around, whether it’s across a room or across a city. In doing that, you expend energy and that can be considered as exercise, even though you may not think of it as “working out”.

For most people, when they think about walking as physical exercise it implies more than just putting one foot in front of the other for a few minutes. Usually there is some duration target you have in mind, or a distance, or the number of steps. There is the notion of getting your heart rate up from your resting pulse rate to something higher, of genuinely working up a sweat.

When I started walking with a purpose, almost any walk became exercise, because I was so out of shape that I would be huffing and puffing after just a few minutes. Now that I am in better shape, I have to set out to exercise through my walking – duration and route become key. A good workout can come when I do at least 60 minutes, with lots of up and down hills, and ideally also carrying a load.

Walking for exercise is also, to a degree, walking with pain. The phrase “feel the burn” is real – when you are pushing your body hard, your muscles are using up the energy stored in your cells, and when you’ve reached the limits of that energy your muscles will let you know it. An exercise walk becomes a real workout for me when I have to grit my teeth to power through, going up a hill or at the end of a long walk. I’ll have a dialogue going in my head – “come on, come on, come on” – and sometimes out loud too if it’s a tough push. My heart rate goes up, and then I know I’m exercising.

There are times when I want some exercise but not a hard workout. In those cases, a walk becomes physical exercise for me as long as it’s more than about 20 minutes in duration. Less than that, and I’m probably just walking to do something – get to the subway or the shops, so over 20 minutes starts to feel like I’m going beyond walking as a utilitarian activity. Having said that, if I stitch together several short walks like that – 10 minutes here, 15 minutes there – so that I get up into the 45-60 minute range, then I’ll give myself credit for some exercise.

That’s the beauty of walking as exercise. It can be a light workout or it can be a hard one. It’s easy to vary up or down, to suit that day’s schedule, weather, and your mood. Picking a great neighbourhood to walk through means it’s not boring exercise either – it’s far more interesting to be out in the sun listening to the birds while you workout than to be grinding away on a treadmill in a gym.

There is another aspect of walking as exercise that I like. What I hadn’t anticipated when I started walking was the notion that the exercise I was getting would often be as much mental as physical. Mental exercise for most people comes in several ways – solving a puzzle, devising a plan, creating a song or play or poem. Walking can facilitate that – by planning a walk, especially a long one, finding an interesting and challenging route; or in staying alert when walking through high-traffic areas or tricky ground; or trying to remember the names of trees, flowers, birds, and insects as I pass.

When I was in my 20s, 30s, and 40s and even into my 50s, my friends would joke about my lack of exercise and I’d reply that my body was a temple where every day was the Sabbath. My exercise was reading a good book. Physical exercise was for those kinds of self-absorbed people who cared more about their physical appearance than their mental fitness.

A health scare changed that for me, and made me take more seriously the need to look after myself. That’s when my lifelong love of walking took on a new meaning and purpose, and walking as exercise became one of my goals. I’ve learned to appreciate walking more as a result, as this new dimension added to those other dimensions about walking that I’ve always enjoyed – walking a meditation, as immersion, as exploration.