As a multi-sport athlete, I track my fitness through swim time trials, burnout lifts, and functional threshold power evaluations that push me to my mental and physical limit. Bryan Johnson , the 48-year-old entrepreneur obsessed with longevity , has a much simpler and less grueling test as a proxy for how your body is aging, which may or may not match your chronological age. Johnson led the exercise at Business Insider's The Long Play event in San Francisco on Tuesday. You can do it anywhere