Another way to think about viva prep: it’s the extra time you get to do a little bit more.
Not a lot of time. Not a lot of work.
The work you’ve already done – research, writing and development – is what gets you through the viva. You’ve written a good enough thesis to share your research. You’ve grown enough in your talent and knowledge to be a capable researcher in your field.
Prep is that little extra to help convince yourself that you’re good enough.