"It was sick [great], it was mad," he remembers now. "I walked out with all those people. That's what boxing can do, capture people's attention. How many people were there, 80,000? Yeah, it captured 80,000 people's attention."
Now it could be 90,000 for a 10pm main event start, if Hearn can persuade the authorities to stretch Wembley to its full capacity: an attendance that would match the record set by light-heavyweights Len Harvey and Jock McAvoy at White City in 1939. "The fight will hopefully be for the WBA, IBF and Ring Magazine belts," Hearn says.
His prophecy back in May 2014 has come true with a showdown that could be Joshua's defining fight and make bouts with the likes of Eric Molina a thing of the past. Hearn says there would be only marquee contests from then on, against Deontay Wilder, the emerging Joseph Parker and perhaps even Tyson Fury.
Kiltschko will be 41 by the time he faces Joshua, 27. Age is against him but there is no questioning his pedigree. Against Joshua's 18 pro wins since winning London Olympic gold, Klitschko can post 64 victories from 68 contests, a heavyweight record 28 world title bouts and an unbeaten run that stretched to 10 years before Fury dethroned him 13 months ago.
In the MEN ring, as they embraced, Joshua told Klitschko: "I've got respect for you, but this is competition." And later Hearn said of Klitschko's warmth towards the best British heavyweight since Lennox Lewis: "Friendly, friendly. That's' a trap Josh mustn't fall into. He has to have the same ruthless streak. But he's a very ruthless individual."
No British fighter has seemed more serenely self-possessed or devoted to his mission on the eve of a bout that could elevate him to global renown. "This is definitely one to get excited about, for sure," he says. "I always said let's focus on this, let's focus on this [smaller fights], but you always knew that fight was bubbling. And to see Klitschko here tonight just makes it real."
There will be no fraternising in the Wembley ring, he promised: "There's different breeds of animals in this sport, but at the end of the day we're all predators, all lions. I think we come together as one, we're representing the sport of boxing, right? I think that's what's important: that mutual aspect.
"I think we get on in that sense. But when we're in the ring it's competition and the best man will be victorious. We're very clear on the respect level but we're very clear on the competitive level as well."
On the question of why Klitschko would want to face a super-quick, agile opponent 13 years younger than him, Hearn chipped in, "I think he's hoping to get you before you're ready." Joshua added: "I don't think he needs the dough, but there's no cap on what you can make. But to own the division again, how long is he going to be around for? Maybe this is to stamp his mark, his last hurrah."
Joshua has already worked out a plan. He says: "You've got someone who wants to throw shots at you round the side and then you've got Klitschko who boxes down the middle. Who have I fought like Klitschko? No-one. That's what makes it interesting.
"Klitschko will present those opportunities because he'll be boxing in front of me. I'll work on those type of things in the next camp." And the sheer scale of the event does not faze him: "It doesn't matter, I'm calm. I've been at Wembley before. It's a fight, man. We'll come together - boom. It's the same rules and regulations."
His mastery of a sport he describes as "risk and reward" stems from an unusually clear and unshakable focus on the job. "From a young age, we were grinding, we were hustling," he says. "Success was always on the mind. This is part of that journey. I stay around the amateur group, who are hungry lions, man. This is all nice, the lights. But it ain't about the money.
"It's about staying focused, your class, your morals, what you're about. What more are you than just boxing? That's why I'm on my grind."
