|  time | miles | minutes/mile |
|-------+-------+--------------|
| 34:43 |   2.9 |        11:58 |
| 56:00 |   5.5 |        10:10 |
| 31:00 |  3.04 |        10:11 |
| 32:15 |  2.77 |        11:38 |
| 33:56 |   3.0 |        11:18 |
| 72:00 |  6.74 |        10:40 |
| 52:22 |  4.62 |        11:20 |
#+TBLFM: $3='(with-time t (/ $1 $2))

#+begin_src emacs-lisp :results silent
  (defun org-time-string-to-seconds (s)
    "Convert a string HH:MM:SS to a number of seconds."
    (cond
     ((and (stringp s)
           (string-match "\\([0-9]+\\):\\([0-9]+\\):\\([0-9]+\\)" s))
      (let ((hour (string-to-number (match-string 1 s)))
            (min (string-to-number (match-string 2 s)))
            (sec (string-to-number (match-string 3 s))))
        (+ (* hour 3600) (* min 60) sec)))
     ((and (stringp s)
           (string-match "\\([0-9]+\\):\\([0-9]+\\)" s))
      (let ((min (string-to-number (match-string 1 s)))
            (sec (string-to-number (match-string 2 s))))
        (+ (* min 60) sec)))
     ((stringp s) (string-to-number s))
     (t s)))

  (defun org-time-seconds-to-string (secs)
    "Convert a number of seconds to a time string."
    (cond ((>= secs 3600) (format-seconds "%h:%.2m:%.2s" secs))
          ((>= secs 60) (format-seconds "%m:%.2s" secs))
          (t (format-seconds "%s" secs))))

  (defmacro with-time (time-output-p &rest exprs)
    "Evaluate an org-table formula, converting all fields that look
  like time data to integer seconds.  If TIME-OUTPUT-P then return
  the result as a time value."
    (list
     (if time-output-p 'org-time-seconds-to-string 'identity)
     (cons 'progn
           (mapcar
            (lambda (expr)
              `,(cons (car expr)
                      (mapcar
                       (lambda (el)
                         (if (listp el)
                             (list 'with-time nil el)
                           (org-time-string-to-seconds el)))
                       (cdr expr))))
            `,@exprs))))
#+end_src