Why McDonald's U.S. boss Jan Fields is moving on

The 35-year veteran takes her fate in stride: "Everyone has a date stamped on their ass and they're the only one who can't see it."

FORTUNE -- For a woman who began her career cooking French fries for $2.65 an hour and never completed college, Jan Fields had an extraordinary run.

But her tenure as McDonald's (MCD) U.S. president ended yesterday, following the company's first monthly sales decline since 2003. Strategic missteps, including a shift to higher-priced menu items that crimped customer traffic, were blamed on Fields, and she was out.

When I called her at her suburban Chicago home on Thursday, Fields declined to share details but made it clear that she'll transition just fine. After 35 years at McDonald's, she has zero interest in taking another fulltime corporate job--she is 57, has accumulated millions of dollars in stock, and reached No. 25 on the Fortune Most Powerful Women list.

Fields grew up in Vincennes, Indiana, the seventh of eight children educated in a four-room schoolhouse. Today, she is a director of Monsanto (MON) and is interested in taking another public-company board position. She also wants to throw her weight behind two causes: women's empowerment and health and nutrition.

Curb your skepticism. While the notion of a McDonald's exec advocating nutrition seems ironic, Fields led the restaurant giant's push toward healthier fruit drinks, smaller French fry portions, and disclosure of nutritional information.

Her personal makeover sets Fields up well for retirement. Three years ago, she changed her diet, took up running, and lost 90 pounds. Even as she completed the Chicago Marathon and ran half-marathons monthly, she kept on eating McDonald's French fries every day--and has no intention of ending her daily habit in retirement.

Her latest stretch goal? The 2013 Chicago Triathlon. "I'm slower than smoke," Fields says about her running pace. "I'm the one picking up the cones at the end of the race." She's even less of a swimmer than a natural runner, she admits. But to prep for her first triathlon, she is taking swimming lessons and simultaneously going to physical therapy three times a week to repair her torn hamstring.

Meanwhile, Field's husband, Doug, who is long-retired, has been commandeering the construction of their new retirement home in Naples, Florida. As fate would have it, he is finishing the project this week. While she oversaw 14,000 restaurants across the U.S., Fields had no time to visit their new home. Soon she'll head to the Sunshine State.

No wonder Fields is taking the end of her career in stride. She says, "Everyone has a date stamped on their ass and they're the only one who can't see it."