It was the catalyst for King to quit his management consultancy job, move back to his home town in Essex and decide to set a world record for running the most consecutive ultramarathons by a man. He set out to run the 50km distance 74 times in 74 days, hoping to raise £74,000 for Headway, the brain injury charity which provided vital support and information services to his family.
He extended it to 79 ultramarathons in 79 days so that he could finish on a Sunday and meet his mother as she came out of church. He then decided to run another, number 80, as a "victory lap" on Monday.
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"The first 21 days my body didn't want me to do this and was rejecting it," he said. "I was having really poor sleep and lots of injuries in the first few weeks and I've never got injuries from running in the past.
"I'm a size nine shoe but at one point I was wearing size 11 and a half shoes because my feet were so swollen. I was sick for a few days, projectile vomiting. Then there was a day where my body just clicked in and said 'you are not going to stop so I will work with you'. Day 80 was my fastest day. I ran it about 50 minutes quicker than normal, in four hours 20 minutes."
Going to extremes is something King has a history of doing. As a teenager he became a gaming addict, spending up to 18 hours a day at his computer until he became the number one player in the world at Call of Duty, the first-person shooter military video game.
"That was quite an extreme part of my life," he said. "I discovered I had an addictive personality and I messed up my education and put on loads of weight. I left that phase a little bit lost.
"I was drinking and smoking while playing computer games and being completely sedentary. It was about as unhealthy a lifestyle as you could get." He left school with two Ds at A-level in business studies and English and weighed 19 stone.
After deciding he needed to change his life he left Frinton and got a job in London as a project administrator for Crossrail. He joined a gym and lost five and a half stone in six months before feeling light enough to try running. He was offered a spot in the London marathon with only two months to train.
"I went from being 19 stone to a sub three-hour-30 minutes marathon runner in less than a year," he said. "I crossed the finish line and a fire was lit in me."
At work he became a chartered project professional and worked in management consulting for PA Consulting for seven years. It was not what he wanted to do with his life and he began taking on extreme running challenges in his spare time.
"I was doing ultramarathons, multi-day challenges, five days up and around Mount Kilimanjaro, and I would be so strong by the end of the week of running that I became intrigued about how far I could go," he said.
"I never loved my job in London and I was looking for a way out. I wanted to do a running challenge but didn't want to do it just for myself. I wanted a bigger reason. When mum got ill I came up with the idea to run one ultramarathon for every year of her life and as soon as I thought up the idea I went and handed my notice in."
He began his record attempt on September 27, raising £74,000 from 2,500 individual donations.
"I was doing five hours a day of running on average and made it an open invite from the start," he said. "In the end 580 people came and ran with me throughout the 80 days. Some people made the journey from Wales and North Yorkshire down to Frinton to join me after hearing about it."
His mother could not communicate for the first eight months after her collapse but the challenge coincided with an improvement in her condition to the point where she could have conversations and understand what her son was doing.
She was there last month on the day he broke the world record, which now has to be verified by Guinness World Records. King has given them all his tracking data, videos taken at the start, midway and end points of each run and a witness statement from his brother who was there for every day of the challenge.
The previous record was set by Krishan Singh Badhwar, a colonel in the Indian army, who ran 60 ultramarathons in a row in 2022.
The record for a woman is 200, set by Candice Burt in Boulder, Colorado, from November 2022 to May 2023. Shannon-Leigh Litt, a criminal defence lawyer in New Zealand, has run more than 700 ultra marathons in a row since January 1, 2024, and aims to stop in 2026.
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King said he hoped to make running his life now. He has embarked on running coach qualifications and wants to play a part in tackling childhood obesity.
"This has made me realise how simple life and happiness can be," he said. "I truly believe now that if you can wake up every day and spend the day doing something you love, amongst people you love, that is enough and what life is really all about."