(CNN) At 91, Queen Elizabeth II of England is the longest-serving monarch in British history, having celebrated 65 years on the throne this year. A glance at her calendar is like entering a Jane Austen novel -- with a multitasking heroine. Her list of duties includes attending banquets and jubilees, appointing officials to obscure high offices, and receiving and entertaining visitors including assorted ambassadors, archbishops, ministers, generals, chiefs of state and various excellencies.
The Queen's hectic schedule demands equal amounts of polite conversation and extravagant dining. So how does she stay healthy and fit?
"The queen's not really bothered about food. All she cares about are horses and dogs," said her former chef Darren McGrady, who worked for Elizabeth and her family from 1982 through 1993.
In fact, he says he is often asked why the Queen doesn't "get big from eating all that opulent food": five-course banquets of dishes with heavy sauces and elaborate mousses, followed by the inevitable glace ice cream.
The nights when she's on her own, she'll stick to grilled or poached fish with some vegetables and salad, but no potatoes or starch.
"That's it. That's all she has," he said. "She's very disciplined like that. She could have anything she wanted, but it is that discipline that keeps her so well and so healthy."
The Queen's simply the kind of person who eats to live rather than living to eat, adds McGrady, and proof can be found in the kitchen itself.
"The chefs and food and kitchens come last. They're still using pots and pans from the 1800s, with the Queen Victoria stamp on them, at Buckingam Palace," he said. While working at the palace, he would ask the Royals, "Don't you want some new pots and pans?"
"No, no, no, we need the money to buy horses and saddles," he'd be told.
"The queen loves to eat food from the estate," said McGrady, who worked at the five-star Savoy Hotel in London before getting his gig at Buckingham Palace. Home-grown vegetables, fish, pheasant, anything off the various estates -- Balmoral, Buckingham and Windsor Palaces -- is what the Queen enjoys most.
"She's also a chocoholic," he confided. "It has to be the dark chocolate, the darker the better. She wasn't keen on milk chocolate or white chocolate."
Does she exercise? McGrady laughs at the question.
"I don't think she has a weight room at Buckingham Palace, but she loves horse-riding and walking the dogs," he said. "She's 91 years young, and she still goes horse riding. She'll walk for miles with the dogs or just around the gardens at Buckingham Palace."
As for her drinking, McGrady mentions all the "silly little pieces in the papers" in which he's quoted as saying she has four drinks a day.
"She'd be pickled if she drank that much," he said. "All I said was she likes a gin and Dubonnet. That's her favorite drink."
It's obvious how this mistaken impression came about, he said: his accent, his swift way of speaking and an imperfect phone connection. The Queen doesn't have "gin in the morning," as some reported, but "gin and Dubonnet" (a spiced aperitif, pronounced doo-BON-ay).
"She doesn't wake up in the morning and have a large gin and tonic," McGrady said, adding that when she does splurge on a glass of wine with dinner -- and this is not a nightly event, he emphasizes -- it will usually be a favorite German sweet wine. "Just in the evening," he added. "She certainly doesn't drink four glasses a day."
Though each has different tastes, the Royals are generally a healthy bunch. "The thing with Prince Philip is, he's sort of into healthy eating, too," McGrady said, describing the Queen's husband as "very military."
McGrady remembers Philip coming into the kitchen one day and asking about dinner. When McGrady opened the fridge, he saw the lamb chops prepared for the staff.
"'Can't we have those?' " Philip asked. "He wanted the staff food for dinner. He was more a Navy man."
Elizabeth and Philip's eldest son, Prince Charles, "was organic before organic was even invented," McGrady said. Each Christmas, the royal kitchen would get a gift from the luxury department store Harrods, a hamper of food filled with treats. One year, Prince Philip came into the kitchen and saw two hampers. He eagerly opened one, asking whether it was the Harrods' hamper.
"Actually, no, Your Highness, this is a hamper from the Prince of Wales," responded McGrady. Seeing plums and vegetables and mushrooms, Prince Philip "slammed the lid down -- 'It's bloody organic' -- and he walked off," said McGrady.
After Charles separated from Princess Diana, McGrady went to work for the princess, who had struggled with bulimia.
McGrady was still cooking "crazy" opulent meals when he joined her household in 1993. "She said, 'Darren, I'm eating healthy' -- and she was patron of 119 charities at the time -- 'you take care of all of the fats in the kitchen, and I'll take care of the carbs at the gym.'
"All my recipes changed when I moved to Princess Diana," he said. He learned to cut back on calories and fat; her preferences included stuffed vegetables. "She'd never eat red meat; she'd only eat chicken or fish," McGrady said.
He speaks of Diana with nostalgia and sadness. They met during the 11 "incredible years" he worked for the Queen, and then he worked for the princess until her death in 1997.
Hear others talk about Princess Diana's lasting impact on CNN's "The Rewind: '90s Edition."
The night of the deadly car accident, "I went to bed, and I had the food for dinner the next day for her return," he said. "It was awful, awful, that next week.
"It was all surreal: going into the garden and seeing the flowers getting higher and higher and all the people and being at the funeral. The whole week was just a daze."
Princess Diana inspired McGrady to leave England and come to America, where he continues to work as a chef and caterer in Dallas.
She is also why he donated the advances and royalties from his first cookbook, "Eating Royally: Recipes and Remembrances from a Palace Kitchen," to the Elizabeth Glaser Pediatric AIDS Foundation.
"Princess Diana inspired millions of people around the world, and she really did me, and in the years I worked for her, I saw the difference she made in people's lives," McGrady said.
One special memory: the day after the 1997 Christie's auction of Princess Diana's clothing in New York.
"The princess came into the kitchen; she was just wearing a white towel and robe. She'd just come out of the shower. Her hair was ruffled, and she just looked stunningly beautiful naturally," McGrady said. Pointing to a piece of paper, she said, "Darren, look at how much money I made for charity just by selling a few of my old dresses." She'd raised about $2 million to fight AIDS and breast cancer, McGrady said.
So when he wrote "Eating Royally," he imagined meeting the princess again and saying, "Your Royal Highness, look at how much money I made for charity just by selling a few of my old recipes."
"My second book, I have to confess -- that one I am donating to my children's college fund," McGrady said. "The Royal Chef at Home," featuring his own versions of American classics, will be published in September.
Even after 20 years of assimilating to distinctly American tastes, his memories of the Queen remain fond.
"It always made me laugh that, you know, one day, the Queen would be at an estate banquet, and she'd be eating off Meissen china from the late 1800s, beautiful hand-painted Meissen china, with gold and silver gilt knives and forks," McGrady said. Another day, she would eat from "a fruit dish -- it was a marble bowl with three horses in gold raised up, holding it, and they're encrusted in diamonds, rubies, sapphires." The gift from the Emir of Bahrain was valued at 50,000 pounds at the time, he said.
"The next day, she'd be at Balmoral, and she'd be on the estate out in the hills, and she'd be eating lunch out of a Tupperware container," McGrady said. "One day they're normal; one day they're royal."