Mark:
The length of your Z2 runs will depend to a great extent on the the state of your aerobic base. If your AeT is high (close to your Ant) then even a 2 hour run at AeT if too much and you need to do your longer runs in Z1. These can be several hours long depending on your event.
On the other hand, if you are aerobically deficient then doing 2 hours (or more) at the top of Z2 will speed up the aerobic base development.
Scott