This is supposed to be the 26-year-old's final "rest day" after a spectacular heptathlon performance in Götzis, Austria, where she took home gold with a British-record-breaking 6,906 points. But as her every last second is accounted for, it's anything but restful.
Apart from the gathered media hoping to catch a word, there are crowds of schoolchildren clamouring for her attention, the bolder ones screeching "I love you, Jess" and most trying for an autograph, a handshake or a hug.
As I traipse around behind her, I start to feel guilty about my part in the jamboree. When we finally get our moment alone together, there is one thing I am dying to ask: doesn't all this get a bit much? "Yes!" she exclaims, before composing a more on-message answer: "I'm definitely more used to it than I used to be. They're just busy days."
Her first answer seems closer to the truth. In the three hours of my watching her being shuttled around, she keeps up the enthusiasm. But between the genuine smiles and friendly conversation, flickers of bewildered exhaustion pass over her face.
For even the most practised celebrities, this would be an overwhelming morning. But Ennis, who when we last spoke two years ago admitted she hated having her picture taken, never set out to be a celebrity. "One minute you're a developing athlete trying to get to the top, then the next minute you do well and win a medal somewhere, and then it's all foisted on you," she says. "You never know when it's going to happen. You don't think about the media side of things when you're a young athlete trying to do well."
In the past few months, Ennis has become one of the most sought-after names in Britain. The petite gold medal hopeful is a gift to any brand, and the ones who have managed to sign her up are all making the most of her time before she goes into pre-Games lockdown. This is Aviva's chance to shoot footage and get press coverage for its "Back the Team" campaign and the company is fitting in as much as possible.
The young athlete from Sheffield is pragmatic about her publicity responsibilities, but she believes things are harder than ever for her generation of Olympians. "Things have changed so much, with Facebook and Twitter," she says. "Everyone is so much more accessible these days: no British athlete has ever experienced what we are experiencing now. It's such a unique situation with the home Olympics. It's not just the sports journalists who want to talk to you and write stories about you, it's all the different journalists from different fields who may not be that interested in what you do, but want to chat about something else – your private life or stuff like that. It's completely different and it's a new situation for all of us... There was no Twitter in 1948!"
Despite the tiring morning, she has no truck with the idea that all this might be more taxing than sport. "The most stressful thing is how hard you have to train and how hard you have to work," she says. "It's the stress of the competition, too – it's not just going out and doing one race or two races, it's seven events over two days and it's such a rollercoaster. I look back at last weekend now and think it was great weekend. I loved it. But, actually, during it, I was like: 'This is so stressful and I'm tired.' It's just really emotional."
"Emotional" is an understatement, considering her season so far. Earlier this year, she thought she had taken first place at the World Indoor Championships in Istanbul, only to discover seconds later, via a live scoreboard, that Nataliya Dobrynska, Ukraine's Olympic champion, had in fact beaten her to it. Forced to go from celebration to defeat in seconds while her face was beamed across the world on television, her response was a lesson in losing with grace. The performance prompted this newspaper to give out "I love Jessica" badges, one of which I have brought along to show her.
"I saw these coming back from Istanbul," she beams. "I thought it was the sweetest thing. I'm going to make my fiancé wear it at all times!"
Her fiancé, Andy Hill, a construction site manager, has waited patiently for her to finish competing before getting around to marriage. And when I ask what she's most looking forward to after the Games, she says without hesitation: "My wedding. That's what I'm really looking forward to. It's such a big year and it's going to be, no matter what happens, stressful – it's already been stressful – the stress of competing and everything that comes with it. I want to make the most of this moment, but I'm also looking forward to going on holiday and then getting married and doing different things."
Her performance last weekend suggests she is, so far, right in her confidence that all this hype won't stop her focusing on the real task in hand: bringing home gold. "I believe that I've got everything to do it," she says, adding with typical caution: "but it's just about whether it all goes right and I get it right on the two days. I'm just looking forward to it. I'm happy with the position I'm in now. I'm ready to go for it."
So will the notoriously modest Ennis finally say that she's the best in the world? Not a bit of it. "I was the best in Götzis, but that's done with now, and the next competition is still to come. London is still to come and things change all the time."
Speaking at the school assembly earlier, though, she gave a glimpse of the steely ambition that is driving her on. One of the pupils stuck up her hand to ask: "How did it feel to win an Olympic gold medal?" The adults – silently, sharply – draw in breath, aware of the gaffe. Ennis takes it quickly in her stride, revealing only the tiniest chink in her confidence: "Well, I've yet to win an Olympic gold medal. But, hopefully, I'll be able to tell you in a couple of weeks."
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1986 Born in Sheffield to Alison Powell, a social worker from Derbyshire, and Vinnie Ennis, a painter and decorator originally from Jamaica.
1996 Attends an athletics summer holiday camp at Don Valley Stadium; meets current coach Toni Minichiello.
2005 Wins heptathlon at European Athletics Junior Championships.
2006 Wins bronze at the Commonwealth Games in Melbourne, Australia.
2007 Graduates from University of Sheffield in psychology.
2008 Foot injury at Götzis forces her to withdraw from Beijing Olympics.
2009 Wins gold at World Championships in Berlin.
2010 Appointed MBE for services to athletics; gets engaged to Andy Hill.
2012 Wins at Götzis, breaking Denise Lewis's British heptathlon record.