Playing for a long time sometimes depends more on mental strength than that of physical. McGrath is the player of that kind who has typically an iron-like mentality and he has already showed his mental power in the past. Ashes is a prestigious series and it is not unusual for a player to have a desire of playing as much as Ashes matches in his career. The main point is that McGrath seems very feet still now, in his 36. So, why not at 40?