Sue Ellen King had circled her retirement date on the calendar: March 8, 2015.

She had worked as a critical care nurse and nursing educator at University of Florida Health (UF Health) in Jacksonville, Fla., for 38 years; co-workers joked that she was there when the hospital’s foundation was laid, which happened to be true. So the send-offs went on for days — parties in the units where she had worked, a dinner in her honor, gifts including a framed photo signed by colleagues.

Ms. King felt ready. She’d turned 66, her full Social Security retirement age. She’d invested fully in the hospital’s 401(k) plan and consulted with a financial adviser. She and her husband, who had already retired, had paid off the mortgage on their three-bedroom ranch. They took a week’s trip to Hilton Head, S.C., to celebrate their impending freedom.

But her retirement lasted just three months. “I’d done all the preparation, except to really think about what life was going to be like,” Ms. King said. Days spent organizing recipes and photos, and lunching with friends, proved less engaging than expected.

So when her handpicked replacement needed a maternity leave, Ms. King jumped at the chance to return for three months. Now back at work in a part-time position she designed for herself, she calls herself “a failed retiree.”