“How do you eat an elephant?” One bite at a time…
It’s an old joke or proverb, depending on how you look at it, but there’s certainly wisdom as well as eye-rolling.
You could never have done your PhD in a week. It takes years of slow, patient work. Learning, discovering, growing. You eat away at the problems of your research one bite at a time.
Getting ready for your viva is similar, but on a shorter timescale. A day of cramming is inferior compared to a few weeks of small tasks, getting ready by nibbling away at a finite to-do list, bit-by-bit. Confidence builds in the same way.
Slow, careful ways that lead to success.