"I didn't want to let myself down. You've got to half look the part," says the dark-haired and affable 40-year-old, laughing. "I've always enjoyed going to the gym and I try to keep reasonably fit, but I've been putting myself through the paces."
This has consisted of high-intensity training, "which has been great for shedding the pounds".
His efforts have paid off - Mistry's looking fit and healthy in jeans and a jumper, having already lost 8kg in the build-up to his debut.
"I've been good on the food front too, cutting out the enjoyable things in the week, but you have to have a cheat day at the weekend."
Despite the gruelling regime, the former EastEnders star who left Albert Square back in 2000, has had a lot of fun getting back on the soap treadmill.
"Coronation Street has always been described as having a family set-up, and as soon as I arrived, everyone was so friendly and helpful. Everyone in the green room sits together, which makes you feel at home really quickly."
The friendly welcome's gone some way to easing him into the hectic routine. "I've never experienced a filming schedule like this one," says the actor, who uses the two-hour commute from his home in London to Manchester to learn his lines.
"I've had jobs where I've had to commute to another country, so this feels as local as can be - and I've lots of time to learn lines!"
His character, Kal, will be introduced to Street residents as an old pal of Gary Windass, played by Mikey North.
"They were together in the army so there's obviously some history there. We'll have to see if the scriptwriters decide to develop their friendship," notes Mistry.
"StuartBlackburn, the show's producer said he wanted me to have fun with it and bring my own ideas to the table, so I think we're going to feel our way with Kal."
And that includes a question mark over where he's from.
"For me, that's great, as it means I can be really free with the character. I've done so many accents over the years that this was a great opportunity to roll with it," says the Scarborough-born star, who is of half-Indian, half-Irish descent.
What we do know is that Kal's been living with family since his wife died, so they could help out with his kids, including a teenage son who's "being quite demanding".
He soon makes friends with Dev (Jimmi Harkishin), who lost the love of his life, Sunita, recently, in the Rovers fire.
"When he arrives, Dev's really keen on getting into shape, and Kal's the man to help him," explains Mistry, who previously starred with Harkishin in 1999's East Is East. "I was quite nervous about coming in, but Jimmi made me feel really at ease.
WE play around with our scenes together. I think he's glad to be doing some more light-hearted stuff."
Soap matriarch Barbara Knox, who fans know as Rita, was also very welcoming, as was Ben Price who plays Nick Tilsley. "He was maybe the first to come up and introduce himself and said it was great to have me on board," says Mistry. He and Price share many scenes, as Kal's helping Nick rehabilitate following his brain injury.
"He's a trainer who will inspire people to be better. He's quite a funny, cheeky little chap, too," the actor notes.
And a ladies' man, perhaps?
"I think to a certain extent he is, yes," admits Mistry. He acknowledges that there are already rumours of him and a certain feisty female getting together.
"Ha, yes, there was talk the other day about Kal getting with Liz (Beverley Callard). Anything's possible! I'll just see where it takes me."
The timing couldn't be better for the actor, who's engaged to his former Strictly Come Dancing partner Flavia Cacace.
THE pair met while competing together in the hit show in 2010, and Mistry popped the question last New Year's Eve.
"I didn't plan on it being then, but it turned out that way," he says, admitting that he understands why relationships develop between dance partners.
"It's quite an intense time, working closely with one other person. You spend hours and hours together learning a new skill," he says.
"Flavia and I had a friendship first and what we have now came after the show. But there are relationships and there are 'relationships'. I mean, we're getting married.
"For me, it was about meeting the right person, so it could have been on that show or somewhere else."
A date hasn't been set for the big day yet. "We're both incredibly busy. I've just started here on Corrie and Flavia's in rehearsals and has lots on. But it'll happen in the near future," he adds. "I think it will be quite a private affair."
The impending nuptials were part of the reason he wanted to sign the soap contract.
"I've done many things in my career and I've travelled a lot," says the actor, who's also appeared in movies including Blood Diamond, with Leonard DiCaprio, and The Guru.
"I'm settling down, so this opportunity to act on a regular basis and be rooted is great."
Growing up, Mistry didn't watch a great deal of TV ("When I was a child I tended to play sport outdoors"), but he did dip in and out of the soaps.
"I've always had a soft spot for Coronation Street. Its tragi-comedy balance really suits my approach, and I've always had a fondness for the show in that respect. But I never thought there would be an opportunity to be in it!"
And the experience hasn't disappointed.
Mistry adds: "All I know is that this is the happiest I've been in my work for a long time."
n Coronation Street, ITV / Mondays, Wednesdays and Fridays