When i did Physics for VCE, the concepts were really hard to understand because you are usually learning and calculating things that are 'invisible' to your eye such as potential energy, electricity and etc.
To overcome this, it is important to refer back to your textbook for relevent theories to consolidate your class materials, and it is okay if you dont understand it at first. I sometimes had to read multiple times to properly grasp what it was saying.
I was dissapointed with unit 1/2 physics because topics like thermodynamics, electricity and etc were not actually relevant to unit 3/4. So, dont get discouraged from physics if you found them hard.
I wouldnt really do any preparation for year 11 during the holidays, as i dont think it would help much... because you dont know what topics you will be taught first and so on.
As for the thick textbook, you will certainly get through them all because you will be assigned questions from each chapter. (Ofcourse some topics are less taught than other important topics)
Not too sure for chemistry, have not done it for VCE.