Anyone who has celebrated a 65th birthday can say she’s been around the block once or twice. Judith Jaffe has been around hers nearly 100,000 times.

Four days each week, Ms. Jaffe goes for a run on the Upper West Side of Manhattan. Her route is simple. As soon as she exits her building on West End Avenue between 75th and 76th Streets, Ms. Jaffe heads north. She runs until she hits the curb, turns west onto 76th Street, south down Riverside Drive, east along 75th Street, and north onto West End Avenue again for a full lap, similar in length and practice to a quarter-mile track. She then repeats the process 18 more times.

Ms. Jaffe started running in rectangles in the late 1980s because she didn’t want to stray too far from her young children at home. Now, more than 25 years later, even after her kids have grown up, moved out and had kids of their own, Ms. Jaffe persists in her repetitive routine.

“What can I tell you?” she said. “I like the consistency.”

Like many runners, Ms. Jaffe, an Orthodox Jewish grandmother and public school pediatrician, sees her regular practice as a rare moment of tranquillity in an otherwise frenzied schedule. One Friday morning in December, she was on vacation from work, but her to-do list was as long as ever. She needed to pick up groceries, call her family in Israel, tidy up the house and cook a turkey dinner for 12 guests, all before sundown at 4:38 p.m., when the Sabbath began. The clock was ticking. Still, at precisely 10 a.m., Ms. Jaffe, barely five feet tall, laced up her sneakers and made her way outside.

As she set off, all of Ms. Jaffe’s stresses seemed to fade away. She ran without headphones or a timer, in silence and in solitude. Left turn after left turn, she circled the block with ease, the Richard Petty of the Upper West Side.

“What I like about running is that it doesn’t require much talent, just stubbornness,” she said.

Ms. Jaffe’s athletic calling did not get off to a sterling start. A self-proclaimed klutz, she was kicked out of ballet classes as a child for mixing up her left and right. Her aversion to exercise only deepened, and for much of her life, housework and trips to Fairway constituted Ms. Jaffe’s most strenuous forms of cardio. Between her work as a pediatrician and the demands of motherhood, there was little time for sleep, let alone sports.

As the years wore on, however, Ms. Jaffe’s health began to falter. She had given birth to four children, and had noticed her bones weakening and her blood pressure steadily rising. Determined to enhance her fitness but unwilling to shirk her maternal duties, Ms. Jaffe struck a compromise: She would try running, though she would never stray more than two city blocks from home. If an emergency ever arose, her oldest child or a babysitter could find her without even crossing the street.

“Maybe it’s a little neurotic,” she admitted, “but it’s comforting as a mother.”

Ms. Jaffe struggled initially with cramping muscles and heavy breathing, barely making it one lap. But she eventually settled into a rhythm. It now takes her roughly one hour to complete her regimen.

“I thank God every time I make it around the block,” she said.

While Ms. Jaffe’s religious devotion helps rouse her to run, it also presents some complications. In keeping with Orthodox tradition, Ms. Jaffe dresses modestly for her workouts. She dons a mid-calf-length skirt over her running tights, long sleeves and a knit cap, all of which can get fairly sweltering in the summer heat. And like Sandy Koufax, who would not play baseball on Yom Kippur, Ms. Jaffe also will not run on the holy day as well as on many other Jewish holidays.

When she does run, however, the neighborhood takes notice. People around the Upper West Side often refer to Ms. Jaffe as “the jogging lady,” and she is regularly stopped at local pharmacies, butchers and clothing stores. Her celebrity is perhaps greatest at Manhattan Day School, the Modern Orthodox Jewish yeshiva across the street from her route. When Ms. Jaffe’s children attended school there, students would line up along the windows facing West 75th Street to cheer her on.

“It was like being related to the mayor or something,” said Rivka Abbe, 21, Ms. Jaffe’s youngest daughter.

As she looped the block that December morning, no fewer than half a dozen passers-by stopped to offer a few words of encouragement or even a round of applause. Edwin Hernandez, a porter in Ms. Jaffe’s building, remains in awe of her discipline and physical prowess. “She never even lets us help her with grocery,” he said. “She tells us, ‘These are my dumbbells.’”

Ms. Jaffe understands why other runners may prefer the scenic views along the Hudson River or the soft tread around the Central Park reservoir, but she has no plans to stray from her sidewalk.

“Maybe,” she allowed, “if they start doing construction.”