But his life was to take a pivotal turn when William went on a family holiday to London around the time of the 2012 Paralympic Games staged in the city.
"At that point I had never even heard of the Paralympics, but it was such a massive event in London and it got massive media coverage," he says. "I watched lots of the action on TV and that's when I recall that some of runners had the same disability as me, and that inspired me to think, maybe one day I can try and run at a Paralympics too."
Shortly after returning to Christchurch he joined the Port Hills Athletics Club where he initially started out as a distance runner. Yet the whole shape of William's athletics future was to lurch in a different direction after he filled out an application form on the Paralympic NZ website and was invited to a training camp in Auckland.
It was here, because of the lack of distance running options for his particular classification, he was encouraged to try the sprints and jumps.
"I was a little bit surprised but I guess the coach knew best, so I went along with it," says the Middleton Grange student. "I actually founds the training easier (than for distance running) and I adjusted reasonably quickly."
From November of that year he also started training five to six times week with his current coach, George Edwards. Undertaking a proper structured training regime his career was then to take a new twist when attending his first international events at the IPC Grand Prix event in Brisbane in March, 2015. It was here the Kiwi was reclassified from a T37 to a T36 - the lower classification representing a more severe impairment - which opened up "a new opportunity to qualify for the World Championships."
At that meet he gained the qualification marks in the 200m, 400m and long jump for the IPC Athletics World Championships in Doha in October of that year. Just a little over 12 months since his first taste of para-sport he was on his way to the World Championships in Qatar.
There in his first taste of major international competition, William performed in his words "reasonable well" to finish sixth in the 400m, seventh in the long jump and tenth in the 200m.
"I didn't really know what to expect," he says. "I had two no-jumps in the long jumps, which was nerve-wracking, but I learned how to keep calm under pressure and deal with such situations."
Returning to training buoyed by the experience he trimmed 0.30 from his 400m lifetime best to record 57.02 at the Porritt Classic in Hamilton in February before a week later at the Canterbury Championships he opting to fulfil a long held desire to compete over 800m. The experience went well, he gained a Paralympic Games B standard running 2:13.5. Then at the New Zealand Track & Field Championships in Dunedin he posted a new national record of 2:12.57 to attain the A standard.
The improvements continued at the Australian Championships has he wiped 0.70 from his 400m PB recording 56.35 and he was rewarded to win selection for the 400m, 800m and long jump at the Rio Paralympics.
"It was a pretty surreal moment as back in 2014 I never would never have thought I would have gone to Rio," he explains. "It was pretty amazing."
Adjusting his training, which is principally carried out on the 400m grass track plus 80m artificial training strip at Hansen Park, to focus more on the 800m rather than pure sprint work he prepared diligently under Edwards' tutelage.
Then on the eve of his maiden Paralympics the Year 11 schoolboy received a boost to his medal chances after the Russian team were banned from the Games as punishment for the country's systematic doping programme.
"The Russian ban changed a lot," he admits. "There was three Russians ranked ahead of me in the 400m and two in the 800m, so that pushed my world ranking up a lot. I knew I had a chance at one medal, but I never expected two medals."
Drawing inspiration from a buzzing New Zealand team, which included the likes of gold medal winners Liam Malone and Anna Grimaldi plus other podium fillers such as Holly Robinson, Rory McSweeney and Jess Hamill - allowed Stedman to performed to a whole new level.
On the penultimate day of action in Rio, Stedman destroyed his national record to claim 400m bronze in 55.69 courtesy of his trademark strong finishing kick.
"I gave that race 110 per cent," he says. "I then collapsed in the tunnel and I threw up a few seconds before climbing on to the podium. So although it felt amazing to win my first Paralympic medal, it was pretty painful as well!"
The next day despite running perhaps 'too conservatively" over the first 500m, Stedman returned to smash his 800m national record of 2:11.98 and clinch a second bronze medal of the Games.
"My coach was not very happy about the way I had I run the race, but he and I were happy with the placing, he admits. "To win two medals was a little but surreal. It is incredible to have two Paralympic medals."
Aged just 16 it is not inconceivable he could have four or five future Paralympics Games ahead of him. The long term goal is the Tokyo 2020 Paralympics Games but before then he is looking forward to a good showing at next July's IPC Athletics World Championships in London.
Increasing the volume of his training and receiving advice from Athletics NZ endurance coach Maria Hassan should aid his 800m. For the forthcoming domestic campaign, William is targeting more PB's and good performances at both the New Zealand and Australian Championships.
But just what has the sport of running given one of New Zealand's brightest teenage talents?
"I love the sport," says William, who plans to later study engineering at university. "It has given me clear goals and as I am a goal-driven person, that's what I really like about it."
