Keep it very general, topics like:
Getting youths or seniors involved in fitness/sport.
Picking the right types of exercises for your needs(explanations of Powerlifting, Weightlifting, Aerobics, Bodybuilding, etc.....)
Bits on fitness trends like kettle bell lfting, Crossfit, etc......
Bits on various supplements, etc.....
Include an E-mail at the end of your column, and let the readers guide you where they want to go:).