I did the even in May of last year during a 3 day cruise on the Wonder. As far as I know the even is held on all sailings and is on Castaway Cay day.
It is very informal, you just meet at the dock the morning of the run and a leader gives everyone the route and runs along. There is no pace requirement, the faster runners go ahead and the joggers or walkers stay behind.
The information will be in your state room navigator the night before. I know there are people who have sailed more recently then me that could chime in with any changes or you can post this question on the Cruise Board. I first found out about it there.