Kyle Thomas’ 100 pound weight loss began one summer night in 2016, when he decided to go for a neighborhood stroll with his father.

Over several weeks, the Canadian tour guide developed a steady walking routine that evolved into a running addiction.

“It wasn’t really intentional,” Thomas tells NBC News BETTER.

He started walking by himself when his father couldn’t join him, and pushed himself to go a little further each day.

“It was just like I was doing it, and it was a little bit of a challenge for myself to just keep going a little bit further,” Thomas says.

Start small

Thomas, 28, says the key to his enormous weight loss was starting small.

At first, walking even short distances gave Thomas aches and pains in his legs. But gradually they started to subside. Slowly — as he got used to exerting himself — he would jog for short spurts.

“I would kind of say, ‘OK, I’m going to run from this lamp post to that lamp post to that lamp post, and just see what happens,’” he says. “And I think back now, and it’s kind of funny because I would be … gasping. I would be panting, and way out of breath, and I would have only gone like 30 or 40 feet.”

He says he followed this unofficial walk-to-run routine about five days a week. It had a sudden impact on his physique. By August, he noticed his clothes were getting loose.

“All the sudden my pants were way too big for me around my waist,” recalls the runner.

The simple routine paid off as his jogging habit snowballed into a full-blown running addiction in just a few months, he says.

Focus on the challenge rather than an end goal

Thomas began going for runs early in the morning, he says. Over time, he increased in distance — from a half a mile, to a mile, to two miles. By December, he was running about four miles at a time, he recalls.

“I was pushing myself to run a little bit faster, run a little bit further every day, and so I was doing more and more all the time,” he says.

Thomas never weighed himself, and never had a specific end-goal as to how much he wanted to weigh or what he wanted his body to look like. Running had simply become a challenge, he says.

Focus on portion size and food quality instead of calories

The runner says he also changed his eating habits. He didn’t pay much attention to the number of calories he ate, he explains. Rather, he focused on reducing portion sizes.

“I might not have eaten horribly, but I ate a lot,” he recalls of his old eating habits.

Instead of counting calories, he ate half the food on his plate. He also started to notice how different kinds of foods affected his body, he adds, and began to adjust his food choices.

“I would also make lean chicken and vegetables, but I didn’t have rice with it or potatoes, all the sudden I would cut out the rice and potatoes because to me they were just filler,” he explains.

Always make time to run

As Thomas continued to lose the weight over the winter months, weight loss started to feel “kind of like a game,” he recalls.

“It was kind of like hacking — hacking my own body to some extent,” he says.

Thomas says he lost so much weight by the summer of 2017 that he was underweight. At that point, he realized he needed to start eating more normal portions of food, he says.

“I’m a little bit easier on myself now that I kind of have a routine of a healthier lifestyle than what I had two years ago,” he says.

Thomas still hasn’t weighed himself at home, but based on doctor’s visits, he estimates he lost around 100 lbs. He no longer has back pain when he wakes up, he observes, and the occasional stomach aches he once suffered have disappeared.