The Team GB trials run from Tuesday to Sunday at the Tollcross International Swim Centre this week, and the Scot knows it is make or break.
“These are the Olympics for me,” she said. “Rio is the one and everything I have done recently has been working towards the trials.
“I’ve always wanted to go to an Olympics. If I produce my best times next week then I know I have a chance.”
The 26-year-old could now be classed a veteran of Scottish swimming, and she has managed to combine her sport while studying for a degree at Edinburgh University.
She is in the midst of a Masters in strength and conditioning, although her studies have been put temporarily on hold as she makes her last push for the dream conclusion to her swimming career.
Coached by Chris Jones at the university, she is one of a clutch of talented breaststroke swimmers in the programme. Corrie Scott, the Commonwealth Games bronze medallist in the 50m, is a rival while Olympic silver medallist Michael Jamieson is a welcome recent recruit and Calum Tait hopes to prove he is among Britain’s best.
“Michael has been great since he joined us last year,” said Johnstone. “It’s nice to have someone even older than me in the group and he’s brought a whole new dimension to training. He’s got so much experience and it’s so good to have someone who has won an Olympic medal in the squad. He’s been there and done it. We all look up to him and he’s always willing to offer advice.”
But Jamieson will have his work cut out to make the team this year. He has struggled to come to terms with losing out to fellow-Scot Ross Murdoch at the 2014 Commonwealth Games and, with England’s Adam Peaty and Andrew Willis in the breaststroke mix, just making the British team will require a world-class time.
Murdoch, Jamieson, Robbie Renwick, Craig Benson, Craig McNally, Dan Wallace, and Duncan Scott are among the leading Scottish men chasing Olympic glory. Hannah Miley who will head the female list, is bidding for a third Olympics and is desperate for the medal that will complete the set. She has mounted the podium at Commonwealth Games, World and European Championships.
Camilla Hattersley, in the distance freestyle, is another Scot whose recent form suggests she has a real opportunity to make it to Brazil this summer. The City of Glasgow 20-year-old won the 400m and 800m at the recent Edinburgh International Meet at the Commonwealth Pool.
“My times over the past few meets have been better than I expected. I’m absolutely buzzing,” she said.