On all results pages, the last column on the right has a dropdown that shows the "Difference" in time to the winner by default.  You can click on the dropdown to select one of several ways to show the relative performance to the the rest of the competitors.

First, all times converted to seconds - and then the following calculation is applied:

Average time     All times added together / number of racers
Median time       The "middle" racer's time in a race with odd number of racers
                            The average of the two “middle” racers’ times, in a race with even number of racers

Percentages

% back               100 * (Your time - Winner’s time) / Winner’s time
% winning          100 * (Winner’s time / Your time)
% average          100 * (Average time - Your time) / Average time
% median           100 * (Median time - Your time) / Median time
% age record     100 * [World record / (Your time * Age handicap)] 

The % age record value is shown only if the the race has been specified to use the "Age-graded handicap" or "Age & Gender handicap".  For details, click on this link:  https://www.runnersworld.com/tools/age-grade-calculator