In her earlier life as a runner, Gail Goldstein was always training for the next race. She qualified three times for the Boston Marathon and ran it twice.

But in Masters swimming, Ms. Goldstein has found what she calls a higher purpose: training for training’s sake. Three or four times a week, she enters a Los Angeles pool with dozens of other swimmers for about 3,000 meters of strenuous effort. “It’s the most amazing workout,” says Ms. Goldstein, a 41-year-old mother of four.

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