for those interested in yoyo, and referee fitness in general there's lot about it in Ep 3 of the ref podcast.
featuring Ross Hanbury mentioned in this thread
The excellent podcasts and RugbyReferee.net site are the brainchild of a RugbyRefs.com alumni, so please support him.
As for fitness Ross Hanbury is an outstanding sport science coach working with the London Society, and is now with the LTA and I think is London Scottish's Match Day Ops Manager.
He ran London Society's pre-season fitness and for the 100 referees recently tested (so not just Group and Premier List L6/7s) the average Yo Yo score was 16.4 - pretty impressive results.