The England international, of course, has been dogged by worrying concussion issues in recent years and had to take a number of enforced breaks from action last season.
Thankfully, he is now symptom-free and looking forward to kicking off the new Betfred Super League campaign next weekend.
However, ahead of Trinity’s big start against Hull FC, Johnstone has revealed another factor that has been restricting his performances: how he has played his entire career “on empty” due to his unorthodox eating habits.
Only the help of a nutritionist has helped the 26 year-old finally see sense.
"I used to go with the mentality that, when I was younger, I could eat what I wanted and stay in shape,” said Johnstone.
“As the years have gone, with my performance, I’ve been playing and after 20 minutes been absolutely knackered.
“I‘ve been thinking ‘why, why?’ and I’d never really thought about nutrition as being that big and key a part even though it is actually huge.
“It is a bit naive actually. I went to see a nutri“tionist and sat down with him.
“ I trained through pre-season and worked on, one, getting my body composition right and then, two, just working on fueling for a game more because when I was a kid I never ate before a game.
“It must have been nerves but I kept it going all the way through to the first team and I’ve been playing on empty and just been exhausted.”
Johnstone, who hopes for a clear run of games to aid his hopes of making England’s World Cup squad, explained what his habits were like.
“I’ve stopped eating a lot of junk food,” he said.
“I was quite lazy. If I didn’t want to cook in the evening , I'd just order something in or go pick something up.
“Looking at it now, knowing I’m supposed to be a professional rugby player, it sounds quite bizarre that that's something I’ve only just done at this point in my career.
“I feel really good now. Obviously, I had a few scares with everything (concussion) last year
“But come the end of the year I sat down with the specialist and he said everything was heading in the right direction.
“He said to have a break from contact but work on fitness
“He thought I’d put on some kilos which was true. I’d been 98/99 kilos, heavier than ever, and he said it will go hand in hand; the more I look after my body, the head will come with it.
“I did my own off-season plan and managed to drop myself down to around 92/93 kilos.
“I’m feeling really good and have managed to train every day.
“I think I only missed two sessions due to having Covid over Christmas. I feel spot on and raring to go.
“Everything I've been doing off the field I hope it correlates.”
Johnstone, renowned as one of the quickest wingers in Super League and capable of spectacular finishes, has worked with his own speed coach to hopefully take his game to a new level.
“I used to do a bit of athletics when I was younger and it’s just something I wanted to get back into,” explained the player, who has also bounced back from two knee reconstructions.
“I’ve got a friend called Mat Cook who I used to play with in the academy and he’s gone down that route.
“I’ve been wanting to get linked up with him the last few years but never really had a chance due to the injuries.
“With being told to take a step back from rugby, and with the transition this year of the S&C and a change around (at Trinity), I just went to Willie (Poching) and said ‘I want to go train with him - is that alright?’
“I wanted to give him (Cook) full reign on it and ask him what he wanted me to do and what do we do from there?
“I just went with what he said and it’s worked well . That’s what we ended up doing.”
Johnstone continued: “I’ve been trying to eat more, eat the right things and doing all the right things off the pitch.
“With the speed coach, it wasn’t just about speed, it was about the way I run and things like that, just to try and avoid injuries and maximise what I can do.
“I’ve been doing those sorts of things but also last year, with the head knocks, I had a few months off drinking just to try and settle everything down.
“I’ll have a heavy drink every so often but now I will try to not do it as often and think about when I am going to do it: the right place and right time.
“I’ve been focusing more on the rugby: is this going to help me, is this not going to help me?”
It is remarkable to think Johnstone has produced such quality performances considering some of his previous preparation so it is scary to think how good he could be now he is looking after himself better.
Are we going to see him even quicker in 2022?
“That’s the plan,” said Johnstone, who scored a hat-trick on his England debut against France in 2018, which remains his only cap.
“We’ll see. It's about working on getting up to speed as well, not just my top speed and things like evasiveness but also more as well as a rehab thing to knock down on the injuries."