Combined with bad eating habits and having two babies, she tipped the scales at 75.5kg - which at her height of 158cm is considered "on the high side of overweight and on the brink of being officially obese".
"I was in a sad place. I remember one time standing in the changing rooms at Glassons and realising that I didn't fit their biggest size. I didn't even recognise the person I was seeing in the mirror," she says.
At the same time, one of her friends was going through treatment for breast cancer while still making an effort to take part in local run/walk series and attend fitness boot camps.
"She had asked me during one of her last chemo sessions to run the Kerikeri half marathon with her, and I had jokingly told her 'I don't do running'."
Her friend lost the battle with cancer last October, aged 32.
"After her funeral I did a lot of reflecting. I vowed to myself to make her proud and make the most of my life. Goal number one was to get fit and healthy."
King saw a six-week-long kickboxing boot camp being advertised by Roundhouse, and mentioned the possibility to a few people. Some of them openly told her she couldn't do it. "That spurred me on enough to walk through the front door of the gym and ask about signing up.
"I literally didn't own a pair of shorts or leggings or even a singlet that I could wear to boot camp. I didn't have trainers either. It felt very odd shopping for sportswear."
At her first boot camp session, King thought she was dying. "I wanted to run, okay walk, off and never come back. But my stubbornness kicked in and I remembered those words 'you can't do it'."
After six weeks, she was hooked - going on to join the gym as a member and doing boot camp continuously for 11 months. "I was starting to see changes in my body and my time at the gym had begun to be my time to think and my time to let out my frustrations on the boxing bag.
"When I really felt like I couldn't keep going I would pretend I was punching cancer in the face."
Her fitness has had flow-on effects.
"My children are eating healthier foods, I have more energy to run around with them, they get to see me role-modelling a healthy lifestyle, and they will grow up knowing how to take care of their bodies.
"My husband has also started running and I have been honoured to inspire a few people to pull on the sportswear and give it a go.
"I feel like the poster girl for 'if she can do it anyone can'."
And it hasn't been an easy road for her. In April, King's dad died suddenly from a heart attack, aged 53.
"I am so grateful that I had the gym as my place to go for physical and mental strength in that time, it gave me resilience."
King's dad was "soccer mad" and would lovingly tease her about her position on the soccer field.
"He started off telling me I was goalie material, then as I lost weight and got fitter he would look at me and say 'ohhh, you're just about a defender now' and then a couple months later 'you've almost made it to mid-field'."
"He never got to see me progress to being an attacker but I reckon he would have to admit that I've made it there now," King says.
Her walk-out song on Saturday was dedicated to him - the Script's If You Could See Me Now.
It was 12 weeks ago that she signed up to fight in Girls'n'Gloves, Whangarei's third annual all-girl kickboxing event at the ASB Stadium in Kensington.
It's been weeks of intense sparring and fitness training in preparation ever since. "I'd never been punched, or punched anyone, before this."
Now her favourite part of training is the sparring. "The empowering thing is the feeling when you do something people thought you couldn't, or you even thought you couldn't.
"I hope we can inspire people to have a go. We're just normal people - mums, students, teachers, beauty therapists ..."
And people that do give it a go are likely to bump into King. "I'm definitely going to carry on training [after today's fight]."
Roundhouse coach Simon Haenga says she is the poster girl and inspiration for the "newbies". But he calls her the Smiling Assassin. "As nice as she is, she beats us up," he laughs.