The viva is a single day when you have to rise to the occasion – but not the first day of the journey that you’re on.
Your viva could be difficult. You can expect to be challenged, but that challenge – discussing your research, your thesis and your ability as a researcher with your examiners – is not the first challenge of your PhD.
It’s not the tenth or even the hundredth.
The viva is one day you have to meet a challenge and succeed. By that day you have a lot of experience of doing just that.