The firefighter and fashion student, 22, was advised by doctors to stay at home in isolation for almost four weeks.
Now fully recovered, Ms Johnston has returned to the front line alongside her crewmates at Galashiels Community Fire Station and is lacing up her running shoes for an epic charity challenge this month.
She is running the 11.2km distance from her front door to the entrance of Borders General Hospital every day throughout May – a total of more than 200 miles.
“I first began to show symptoms of Covid at the end of March, and followed the advice to stay at home and completely self-isolate,” she said.
“I got progressively worse, and had a few pretty bad days, so I called the NHS 111 helpline.
“As I was still able to breathe well enough by myself, I was told not to go hospital, and instead continued to speak regularly with NHS nurses and doctors at the BGH.
“I had a mixture of bad fevers and a tightness in my chest, would wake up completely exhausted, and even the smallest thing such as taking rubbish out to the bin was a real struggle without stopping to catch my breath.
“In the end, I waited it out over 26 long days at home by myself.”
She added: “I was overwhelmed by the support I received – from the NHS 111 nurses and doctors at the BGH, the guys at the station who would check on me, and my ‘Borders family’ down here who reached out.”
Ms Johnston’s charity challenge also comes after she was treated at the Melrose-based hospital for a fractured elbow and concussion following a bad cycling accident last summer, when she was knocked off her bike.
The Elgin-born firefighter is raising money for the Friends of the Borders General Hospital’ charity – which funds vital equipment and services at the region’s largest hospital – and smashed her initial target in just a few days.
Area Commander Stephen Gourlay, the Scottish Fire and Rescue Service’s local senior officer for Midlothian, East Lothian and the Scottish Borders, said: “Flora is one of very few full-time students serving as a retained firefighter anywhere in Scotland, and is highly regarded by her colleagues and community. We are proud to call her one of our own, and to support her fundraising challenge for an important and valued local charity here in the Borders.”