(defun my-total-exercise-hours ()
  "Return the sum of all exercise durations in hours."
  (format "%0.2f"
          (/ (let ((total-duration 0))
               (maphash (lambda (day duration)
                          (setq total-duration
                                (+ total-duration duration)))
                        (my-exercise-log))
               total-duration)
             (* 60 60))))