He did not know it then, but the situation was so grim his wife Claire – then pregnant with their second son – was warned to keep the phone close at night.
"I'd been sick for two or three weeks – I just couldn't shake it," Mr Johnston said.
"Obviously I was sicker than I realised. I had severe stomach pains and the doctor sent me straight to Royal North Shore Hospital, where tests showed my kidneys were shutting down and I was going into septic shock.
"Then my liver started shutting down, and my breathing became quite laboured, so they decided to put me in an induced coma where I stayed for eight days while I battled the infection."
When he came out of the coma, the Castle Cove man could not walk, sit up or feed himself. It was three more weeks before he was well enough to go home, with the cause of his life-threatening infection still a mystery, and three months before he could return to work.
The now-36-year-old father of Angus and Edward had decided it's time to give back to the people who saved his life.
Not only did he raise almost $5000 for the intensive care unit at Royal North Shore by running a marathon, he is also throwing his weight behind the annual RNSH Christmas appeal.
"At the time I was so sick the support from friends and family was very important - but the work the hospital staff did, how well I was looked after, and the care my family was given is something I will never forget," he said.
"It was very tough to accept I needed to be looked after. I felt like a small child, and I found that tough. But I made steady progress and that was very helpful, mentally.
"The staff provided fantastic moral support. The way they could describe to me what I had been through, and the caring way they dealt with me, even when I was feeling very frustrated, was fantastic.
"When you are fit and healthy you don't realise the small things that can make a difference to people when they are sick, and you probably don't appreciate how many sick people are out there.
"I really want to encourage people to support this appeal because money goes a long way to helping horrible experiences be a little easier."
Donations will be used to purchase a replacement $80,000 heart balloon pump which helps patients who are recovering from heart surgery or a heart attack. To donate to the "Share Your Heart" Christmas appeal, please go to www.kolling.com.au or phone 9436 0610 or SMS 'GIFT' to 0455 121 121.