"I don't do a lot of sitting," Nancy Mimms told me. At 73, the lifelong dancer, former competitive bodybuilder , and fitness industry pro likes to keep busy. The day of our interview, she'd had a busy morning, teaching group exercise classes and training clients at the Bay Club, then working in her garden. After our interview, she said she was going to go for a walk, to enjoy the unseasonably warm weather in San Anselmo, just across the Golden Gate Bridge from San Francisco. When she's not at