The prolific winger was named man-of-the-match following his stunning exploits in the 44-6 victory over France, crossing for his treble inside just 13 first-half minutes.
But Johnstone, 23, said: “The nerves were really bad at some points beforehand.
“It felt like a very long day and I just wanted it all to come quicker.
“I tried calming down, though, by treating it like a normal Super League game, something I do week in, week out, and it worked.
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“It relaxed me and I didn’t let the moment get ahead of me.
“I couldn’t have written it any better; I just wanted to go out and play a standard game and try and fit in with the boys but to get that (a hat-trick) is what dreams are made of.”
Johnstone got the first chance to claim the vacant left-wing spot that has opened up due to an injury to England’s record-scorer Ryan Hall.
He certainly made his mark although coach Wayne Bennett suggested afterwards that Tommy Makinson – the St Helens wideman nursing a knock – would come in for the first Test against New Zealand in Hull on Saturday week.
“I feel like I’ve done everything I can and put my foot in the door,” said Johnstone, with Huddersfield Giants’ Jermaine McGillvary nailed on for the right-wing slot.
“Hopefully, Wayne is happy with what I’ve done and he might give me a chance. He gave me a handshake and said ‘well done’. I’m really happy with that.”
His impressive debut capped a fine campaign for Johnstone, who scored 24 tries and earned his first Dream Team selection.
That came after missing most of last season due to a knee reconstruction and the player – born in Germany but raised in Leeds – admitted: “It’s incredible.
“I’ve had some real low times these past 12 to 18 months so to get in the changing room, see my shirt and go out there with the boys is so rewarding. It made it all worthwhile.”
His treble helped put England in command 38-0 at the break, although France finally stirred to make more of a game in the second period.
“We trained really well and the boys came together really well,” added Johnstone.
“France came out and were flying off the line trying to put some big hits in. But we just controlled the game, played at our level and it paid off.
“They put in a good stint, though, and the score doesn’t fully reflect the effort they put in.
“We showed some things are clicking but we’ve still stuff to work on ahead of the Kiwis.
“We’ll work on that when the rest of the boys (nine Grand Finalists were unused) come into camp next week.”
Bennett says Johnstone’s display earmarked him as a player for the future and certainly for the 2021 World Cup.
“If he doesn’t make it (this time), he won’t be far off,” he said.
“He’s someone I’ve noticed for the last 12 to 18 months and someone I’ve been waiting to give an opportunity to. It’s come along this year and he’s certainly got a future on the wing for England, but it may not be this year, we’ll have to wait and see.”
Johnstone’s Wakefield team-mate Reece Lyne also impressed at centre on debut with Bennett describing him as “project” who “certainly has some ability on his performance.”