Carole Alter slips a cassette into a boombox and soon a didgeridoo and tribal tempos fill the Valley Room of the San Geronimo Community Center. It’s Monday morning and Ms. Alter’s class, Exercise for Altacockers, a Yiddish word for “old farts,” is underway. It’s designed as a synergistic combination that weaves together dance, exercise, yoga, energy . . .