“My husband would like me to go running with him, but I don’t want to,” says Marla Centralla, a mother of two. “It’s not that I don’t like running or don’t love my husband. I just want to be alone!” Some people, sports medicine experts have discovered, prefer to exercise on their own. They would much rather ride a stationary bike in a quiet corner at home or perhaps walk or run alone. Failing to take into account this personality trait, experts say, causes a premature end to many an exercise program.