When Kim’s new grandbaby comes to visit, she doesn’t think twice about carrying him up and down the stairs. Between visits, she carries a 15-pound weight on those same stairs — just to stay ready. That’s the kind of practical, purposeful thinking that defines her approach to fitness at 70. Since joining a gym with her husband, Rick, 74, last year, Kim has lost 25 pounds and dropped a clothing size. But those numbers aren’t really the point. “I’m more fit now than I was 10 yea