That’s the viva. At and after submission there’s still lots to do, but at and after the viva there’s hopefully only a little way to go. Still work, but not too much.
The viva is the beginning of the end of your time as a postgraduate researcher. If you’re tired at this stage remember that there’s not far to go, not much longer you need to keep going.
Prepare for it, enjoy it if you can and finish the work you started.