NIKKI ROBBINS, 60, credits her great shape to a life-time of devotion to fitness. She was involved some years ago with the Hollywood-fitness pioneer Vic Tanny. Her father was an animator behind the creation of Yogi Bear and Beanie and Cecil--she remembers when her mother sewed the puppets out of a towel. She works with seniors at a Southern California center. "What I get out of life, I give elsewhere. "