On your first day of viva prep you don’t have to push yourself to work at maximum intensity.
Maybe make a list.
Read a chapter.
Check a regulation or two.
You don’t have to start and maintain an exhausting pace of work and progress towards being ready.
You don’t have to do that from day two either. Or day three.
You can build up your prep and yourself. Take your time, plan your prep, build yourself up for the viva. A little work every day for however many weeks you need to not be stressed by your schedule. That’s all you need.
Build up to the viva rather than risk wiping yourself out with too much pressure.