Density
Thousands of hours of work, spread out over hundreds and hundreds of days.
How much thinking? How much practical work? How much reading?
Now all squashed into several chapters, a few hundred pages maybe.
Your thesis is dense.
In preparation for your viva you need to dig in to it.
Read what you wrote. Reflect on what it means. Review what you need to so you feel ready.
And remember that dense substances are often very valuable.