It is not these extraordinary times that have made Joe Wicks. He was already huge, initially from Instagram cooking videos that lasted just 15 seconds. Rudimentary but engagingly zany, they attracted a substantial if hodgepodge fanbase from body-conscious zoomers to middle-aged mums. So when he brought out the cookbook Lean in 15, five years ago, his publisher said she would be delighted to sell 70,000 copies, which roughly amounted to one per Insta follower. Instead he sold 700,000, to become the second biggest seller in the cookery hall of fame, below Jamie Oliver.

This week, though, his live workouts on YouTube have put him in a different league. Every morning at 9am, he is running a “PE lesson”: an exercise class for kids that you can do in your living room. It sounds like a simple idea, but now every child in the country (and their mum) is talking about it, his name forming a compound noun with his chosen title: “Yes, I’ve done my PE homework, I’ve done JoeWicksBodyCoach.” Kids love the kangaroo hops and parents love the fact it gets their children burning off excess energy when they are trapped inside.

Skyping me on Tuesday afternoon, Wicks runs the numbers: “On day No 1, we had 806,000 households streaming it. Today it was 954,000 livestreams; 3.7 million people have watched the first video since yesterday. It’s just growing, I’m over the moon. I feel quite overwhelmed by it.” He has been up since seven, and so busy he has only had time for a piece of toast. This is a salient detail from the nation’s ultimate dietary role model, whose MO is to show not tell; normally his lunch is way more balanced.

He is keen to stress that this is no overnight success. The Instagram videos built up over two years, from 2014 when he was an obscure and incredibly bouncy personal trainer, shaking pans about, being cute about broccoli (“baby trees!”) and wild about rice, to 2016, post his first book, when he had a million followers. He is quite down on his early broadcasts, describing his voice as monotonous and critical of the lighting. “No one that probably really loved cooking, like really skilful chefs, would have watched those,” he says. “They were for general people who wanted to know where to start.” But there was something distinctive about them, an unstudied inclusiveness

It was in 2016 that he started visiting schools, up and down the country, trying to embed a passion for physical jerks. He wanted to teach the world to star jump; he wanted a TV show, like Jamie Oliver’s School Dinners but for exercise. He could not get any takers. But that was then. Yesterday, he was in discussions with Channel 4 about streaming his online workouts while the Sun was poring over his videos, trying to spot evidence of a lavish lifestyle from the shots of his living room in Richmond, Surrey.

“It’s a very modest house,” he says. “People just focus on the stupidest things. I mean, look: it’s a nice kitchen, but it’s not anything fancy. It’s because I won’t do an interview with them, that’s why.” Hmmm, the last person who said they’d never do an interview with the Sun was a Labour leadership candidate, I say a little slyly. “I don’t do politics,” he says, laughing; his mind is on a larger goal. “I want to be the person who completely transformed the health of the nation.”

I do not think Wicks has a particular fitness secret: his workouts are successful because people love him. This is the mystery of the man – why is it that people, across generations, like him so much? It is not a question that you can ask directly. You have to go back to the beginning.

A disruptive and naughty kid, “a bit of a clown”, he says, Wicks was always into sport, football, cross-country, anything where he could blow off a bit of steam. The defining moment of his teenage years was when, at 15, a group of his schoolmates was taken on an immersion day at St Mary’s University, in Twickenham, London. “I remember looking round the bus, and it was all the naughtiest kids, the ones who were always in detention. I think they must have wanted us to see how it could be different.”

On the way home, he rang his mum and announced that he wanted to be a PE teacher – and he did start as a teaching assistant, before becoming a personal trainer. It was not an obvious trajectory, he says. “I had quite a chaotic home life, it wasn’t stable, my diet wasn’t great. I was never an overweight child, but I had behavioural issues. I think that was linked to my upbringing and not having a great start with my nutrition.”

His mother was 17 when she had his brother, 19 when she had Joe; his father was in and out of rehab for drug addiction. “There was a lot of shouting, a lot of doors slammed, we didn’t sit down and have dinner together.” And yet, he says: “One thing I had more than anything, I had love and support from my mum and dad. My mum used to say: ‘I don’t care if you become a dustman or a doctor, you can be who you want.’” It is a powerful and unusual stance, and it means he really gets imperfection – human frailty, too much sugar, not enough lunges – he gets that you can make poor choices without being a bad human being, he is palpably non-judgmental, without ever saying: “I’m not here to judge you.” I think viewers can smell that, at any age, even through their smartphones.

There is also, of course, the impossible to ignore eye-candy element, which meant in his early career he was often seen on the covers of fitness magazines displaying his washboard torso. He is quite uncomfortable about that now. “I am a dad and I am married and I’m very public about my love for my wife [Rosie, who used to be a model]. So the attention has shifted, it’s less: ‘We love him because he’s got abs’, the heartthrob days are over.”

Anyway, he says: “I’m 34 now, receding hairline, I’m not as lean as I was.” Which is stretching things a bit, since to the untrained eye, he looks the same, only more famous. But he is certainly serious about his family. “It’s a nice calm feeling, since I got married and had kids [a daughter of two, a three-month-old son]. I’m not chasing anything, I’m not rushing to get anywhere, I’m really content with what I’ve got. When you’ve got a nice house and you’ve got your kids and you’ve got your holiday, what more do you need? There isn’t much more that will ever make you happier than that.” If you have to pretend to be going bald to damp down the passions of a whole nation who refer to you as “Juicy Joe”, I guess that’s a price worth paying.

Anyway, back to the fitness; there is a good reason to start with younger kids. Before coronavirus hit, when he was still touring the country, “I went to a secondary school, and they were already a little bit too cool: ‘I’m not training with him, who is this dude?’ My whole philosophy is to get kids exercising at a young age, which you can really create with role modelling. ‘My mum’s sweating, she’s out of breath, she’s laughing.’”

Take a sedentary teen, who is into their devices, and good luck training them, he says. This might be disheartening news if you already have a house full of sedentary teens who are all into their devices, but he continues persuasively, and by the end of it I’m convinced that you probably could persuade even older kids, so long as you directed them his way and didn’t try to role model it by sweating yourself. “It’s just about happiness,” he says. “I’ve stepped away from exercise to look good, exercise to lose weight. True motivation comes from how exercise makes you feel. If you’re very demotivated, you’ve got to remember that the sense of achievement comes at the end of the workout, not at the start.”

As much as Wicks proselytises fitness, he is careful to show his flaws. “People like the fact that I’m out of breath when I’m training, or I’ll have a blowout, I’ll do a little choccy run. I make mistakes, I’m constantly learning.” Even though he has had the ultimate social-media start, born on Instagram, raised by Facebook, he hasn’t fallen into the trap of presenting an idealised version of himself, then having to meet the expectations he has raised. “You can put on a front, but people see through that very quickly, and they disengage.” He never sounds censorious, even while he is describing his meal plans for the day – oats and berries, omelettes and salad, vegetable curries in the evening – and sketching out the unutterable wholesomeness of his mood management.

“There are days when I wake up feeling flat, I don’t know why,” he says. “I’m not unhappy but I’m not happy. I know, if I feel like that, I’ll go and do a workout and it will lift my mood.” His workouts sail forth on this tide of infinite enthusiasm, which it is hard not to get swept up in. “I don’t know if you believe in the energy and the secret?” he says at one point. I have no idea what this means, so I say that yes I most definitely do. “I do believe,” he elaborates, “that the more you put out into the universe, the more that comes back.”

If the positivity is a bit relentless, it is definitely not unthinking. He allows in anxieties: small, medium-sized, vast. He worries whether he is as patient as he possibly could be at the dinner table, about how personal trainers are going to get through this hibernation period when they are all self-employed. “They really need some love. If you can’t find what you want from my channel, type in yoga for the elderly, or pilates for pregnancy: there’s millions of trainers also doing really good videos.”

Since he had children, “I have days when I start thinking about the Earth, about pollution, about the sea, about the economic stuff. I’ve started to think about what it’s going to be like in 50 years, and I didn’t used to think like that at all.” It has made him more driven, he says, but not for money. “I’m proud of what I’ve achieved, yes. You have to understand I grew up on a council estate, and back in the day you had tokens for your lunch at school. I didn’t come from a background of wealth, but it wasn’t something I talked about, I still had a good life. And I’m still not motivated to be financially successful. I don’t look at the numbers, or think about financial gain. What makes me feel good is reaching people, trying to change the culture so that parents want to exercise with their kids.”

I think back to how stung he was when the Sun picked over his living room for signs of poshness. It actually doesn’t look posh at all. He has just moved all the furniture out so there’s room for press-ups, and there is a map above the mantelpiece. “It’s a plastic map!”, he says, mock-scandalised.

He asked me at the start of the interview whether I was Zoe Williams the media doctor, and I said no, but I was once invited to give a keynote to the Royal College of GPs, and because of my weird ego, I exchanged loads of emails on my thoughts about general practice, before my Mr said: “It’s possible they don’t mean you.” He has a version of that story, and it’s much better. “My brother got invited to do a speech at an event at YouTube for the Who. He’s a massive fan of the Who, he’s been listening to their music for years. The day before, he thought: “Why do they want me there?” And it was only the World Health Organization.” It’s a signal of how big Joe Wicks was, even before the online workouts, that the WHO wanted his brother, Nikki, who is also his social-media strategist, to talk through how best to improve the world’s health.

Even while he is in no way blase about the lockdown – worrying not just about personal trainers, but about people with businesses, families in debt, how we will adapt to not being able to get the ingredients we’re used to, the stuff we thought we relied on – he is constitutionally oriented towards the bright side. “I think the real value that people get from my YouTube videos are these questions. Are you going to feel good today? Are you going to sleep better tonight? Do you want to give yourself the opportunity to feel the best you can?”

Well, OK then. If you put it like that …

• PE With Joe streams live at 9am UK time on his YouTube channel.