This is rare in life, perhaps unique. Simply focusing on some measurable element of your physical nature can prevent you from becoming a “Dow Joneser,” someone whose self-worth is dependent on things largely outside of their control. Job not going well? Company having issues? Some idiot making life difficult? If you add ten laps to your swimming, or if you cut five seconds off your best mile time, it can still be a great week. Controlling your body puts you in life’s driver’s seat.