Breaks are an important part of the viva process. For length, for comfort, for medical reasons – there are lots of situations where a break is needed. It’s right to expect your examiners to offer them; it’s right to ask for one if you need one.
Concerns about long vivas often stem from a candidate wondering how they could perform well over long periods of time. Breaks help. Perhaps lots of worries about “what happens in the viva” follow from missing pieces of information.
You can ask for a break if needed, so that aspect no longer needs to be a worry.
What other worries do you have? Who could you ask to help you with them? What could you do?
How can you break up your concerns so that they become something you can resolve?