… is the artihmetic mean of all exercise durations
\[\begin{equation*} \frac{\sum_{d \in D} f(d)}{60 \thinspace |D|} \approx \boxed{42.29} \, \text{minutes}, \end{equation*} \]
computed by
(defun my-average-exercise-duration () "Return the average exercise duration in minutes." (let ((log (my-exercise-log))) (format "%0.2f" (/ (let ((total-duration 0)) (maphash (lambda (day duration) (setq total-duration (+ total-duration duration))) log) total-duration) (* 60 (hash-table-count log))))))