The Xystos was another structure used for the Games. Malalas relates that this was built (or rebuilt) by Commodus (180-192AD). This is described in some commentaries as a covered track used for sports. In a footnote A.F. Norman suggests some sort of linkage with the Theatre of Zeus and references Roland Martin in Festugiere but my reading of Martin quite clearly states the Theatre of Zeus was in Daphne, moreover Norman attributes Martin as claiming that the Xystos and Plethron were in some way connected with the theatre. Maybe our French is faulty but R. Martin says in reference to the Plethron "où il est bien distingué des deux theatres". That sounds like the contrary view.
Both of these structures were in or around the Forum, which is usually named in honour of Valens, for his rebuilding activities.
I have uploaded Richard Foerster's edition of Libanius' Oration X: "On the Plethron" and it is available here: