The viva is, in some ways, a bit like a train journey…
You have your ticket (thesis), you know your destination (graduation) but it’s right that the conductors (examiners) check you’re supposed to be there.
There’s clear norms about what’s involved. It takes time. It’s rare for something to go seriously wrong. If sometimes it might take a little longer than expected it can be coped with. Even if it was cancelled, there would be another at some point.
Thankfully, unlike a lot of trains, the viva is not too crowded and you have space to stretch out a little and think.
You should probably still keep your feet off the seat.