The McDonald’s (MCD) boss behind the healthy upgrade to its U.S. menu is practicing what she preaches: She recently lost 90 pounds.

Jan Fields, who started at McDonald’s 33 years ago cooking fries and is now the fast food chain’s U.S. president, was soon to turn 55 when, she says, “I woke up one day and said, “Oh my God, how did I gain this much weight?”

Like millions of her customers at the 14,000 restaurants she oversees, Fields added her weight gradually–“10 pounds at a time: 10, 10, 10 and all of a sudden, I looked and I said, ‘Oh, my God, I’ve gained 90 pounds. How did I ever do that?”

Of course, McDonald’s tops critics’ hit list for compounding America’s obesity epidemic, but Fields insists that her own culprit wasn’t food but lack of exercise. “I didn’t exercise,” she confesses. “I worked all the time, went home and went to bed.”

Fields, No. 25 on Fortune‘s Most Powerful Women list, has always been easy to underestimate: The seventh of eight children raised in tiny Vincennes, Indiana, she grew up skinny and dreamed of being a nun, then a lawyer. But she met a guy, married young, had a daughter, and was driving to an interview for a secretarial job at a construction company when, at 23, she stopped at McDonald’s for a Coke and spotted a “Now Hiring” sign. It was, in fact, the first time Fields ever ate in a McDonald’s. She and her husband, who had enlisted in the Air Force, were so poor that they couldn’t afford to eat out.

Accepting a job on a whim and starting at $2.65 an hour, she went home that first night and cried — “a disaster” until they moved her from cooking fries to serving customers at front counter. “Never quit over one thing or one person,” she advises. Rising rapidly through the ranks, she was propelled by a remarkable discipline and a down-home, inspirational leadership style.

Along the way, Fields was working so hard that she never had time to get a college degree or take care of herself. Until she visited a nutritionist and committed to do something about her weight gain. “I started with going to the gym and getting on a treadmill,” she says, noting that all you need is a walking path to begin a fitness routine. “I didn’t hire a trainer.”

She started drinking a lot more water. And no soda, not even the diet kind. Fields still eats at McDonald’s every day–but different foods and smaller portions than she used to. Breakfast is McDonald’s oatmeal or an Egg McMuffin or a fruit and yogurt parfait. Lunch, at the McDonald’s next to her office in Oak Brook, Illinois, is usually a grilled chicken sandwich or a cheeseburger, with a few French fries and iced tea.

Dinner is not at McDonald’s — she does get away from work. Typically, she has pasta, or, she says, “I’m big on tomato mozzarella salad. That’s my favorite all-time food.”

The focus on healthy eating has been good for Fields — and the company. New products like smoothies and oatmeal have helped expand McDonald’s customer base and profits, and the stock recently reached an all-time high. Meanwhile, McDonald’s announcement last week that its restaurants will serve fruit or a vegetable with Happy Meals and cut the portion size of French fries signifies “our first step toward creating serious change,” Fields says.

Fields, who loves to mentor young people, contends that her story proves that people “can do anything that they can set their mind to.” And what, as she turns 56 this month, is she setting her mind to next? The Bank of America (BAC) Chicago Marathon, on October 9. Though she never ran a mile until this past February, Fields is now training vigorously. On Sunday, she emailed me: “This past weekend, I did 13.1 miles,” she wrote. “It was so hot (got to 90), it took me 3 hrs and 12 minutes. So I will not be qualifying for any Olympic trials any time soon…but I made it :)”

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