At 17 years old, he was paralyzed from the waist down. 3 years later, he competed in a marathon.

Mason Branstrator has always been an athlete, but three years ago, at just 17 years old, his active lifestyle changed forever.

"I was going for an ordinary day of downhill skiing, but this time, it was a lot different," Mason, an experienced skier who frequented the slopes in his home state of Minnesota, told CBS News. "I remember thinking right as I left the jump, 'I'm going a little fast.' And that was the last thing I remember before everything went black."

Mason broke his T-12 vertebra, which affects lower body functions. He was paralyzed from the waist down. As an avid runner, soccer player and skier, he couldn't wrap his mind around the fact he may never walk again.

"In the ICU, multiple doctors came into my room and said, 'Mason, you might not be able to walk again.' And in the eyes and mind of somebody who was 17, I had no idea what was going on. That was devastating," he said. 

He spent nine days in the ICU in Duluth and was then airlifted to Craig Hospital in Colorado — which Mason said is one of the top hospitals for paralysis rehabilitation — where he spent about four months in rehab. He has returned to Craig regularly to receive more treatment and did regular physical therapy sessions at home, gradually increasing the amount of steps he could take and building his muscles.

Mason says his parents raised him to focus on the good and while at Craig, he became more optimistic. 

"The first time I got in a wheelchair in rehab, I felt so free," he said. "In this really hard situation, you're given freedom and independence. And I've always been so grateful for my wheelchair. And people look at the wheelchair and say, 'That sucks. It's awful to be limited to using a wheelchair.' But this wheelchair doesn't limit me. It's actually my freedom."

After rehab, Mason returned home to Minnesota where he became homecoming king. And after graduating high school, he decided to go back to Colorado, enrolling at the University of Denver, which is near Craig Hospital – a place he now visits not as a patient, but as a mentor. 

Still, he said being in a wheelchair is not easy and it takes him longer to do most things people take for granted, like getting in his car, which he showed us during our time visiting him at University of Denver. Mason has to get out of his wheelchair, balance himself against his minivan – which is equipped to hold his wheelchair in the back seat – and hoist the chair in himself.

Then, he slides into the front seat, which he drives using a joystick next to the steering wheel, rather than break and gas pedals. 

He uses a walker to get out of bed and uses a vertical board that he straps into so he can get out of his chair and be upright for a few moments a day. He spends extra time finding wheelchair ramps in and out of buildings and maneuvering stairs and escalators, going down backwards in his chair while holding on to the railings. And on several occasions, his dad has carried him on his back up flights of stairs when there is no elevator available. 

All of these challenges haven't prevented him from playing sports and living life even though he can only take a few steps here and there.

"Around a year and a half after my accident, I had come to the point of realizing that walking was not going be a very functional thing for me to keep pursuing and it was time to look into everything else the wheelchair world had to offer. And I had all of these people giving me offers to come play adaptive sports and try them out but I was pushing it back. And I finally realized it's time to just live life."

Mason said when he started to say "yes," that's when his world changed. He works out in his wheelchair, and plays adaptive tennis and basketball in his chair as well. He swims using his upper body and has surfed waves in Santa Cruz, California while strapped to a kayak. 

And when CBS News met him at University of Denver, he was training for a marathon, entering the wheelchair race. The university allowed Mason to keep his stationary bike, which he powers with his hands, in its spin classroom. He also has a hand-powered bike that he rides along trails near the college to train. 

Last month, he returned to his hometown to compete in Grandma's Marathon, an annual race for nearly 50 years in Duluth, Minnesota. He came in at 2:03:52 minutes.

After his accident, Mason found a way to help others. 

"When I was 17, on my way home from a fishing trip with my dad, I remember telling him I had this dream of helping people, but I had no idea how I would get there" he said. "Two days following telling him that, I woke up in the ICU and the doctors informed me I was paralyzed."

He started sharing his journey in Instagram to connect with others who had physical disabilities, and now has nearly 270,000 followers. "And then, it just kept evolving and more people caught on to my story — people who weren't paralyzed started being inspired. And I just loved doing it," he said. 

Mason said he shares things that are ordinary to him — like how he gets in and out of bed, or how he jumps in a pool or gets out of a boat – but he's seen how sharing these normal activities can inspire countless people. 

"There were a couple of moments where people reached out to me and said, 'I used your method to do something and it really changed my life, changed how I interacted with the world and changed my perception of this new normal,'" he said. "There's been multiple moments where I realized my dream of helping people has come true and I'm living it out right now."