Hi, I would say that these subjects are just as demanding as any other, and the only indicator of whether or not you'll truly be able to succeed is whether you enjoy the subjects or not. A couple of people in my physics class only took the subject as they didn't know what else to put and ended up switching to a different subject within the first couple of weeks - although the content we were learning was nowhere near as difficult as what we'd encounter in A2 physics, they realised pretty early on that they lacked the motivation they needed to keep them going for two years. A massive tip is to focus on yourself and yourself only. There will be people who seem to revise day and night all year round and there will be people flaunting their test scores, but as long as you are doing what you need to be doing, you'll be fine, and you may even find that A Level physics isn't as hard as people make it out to be!
Subject-specific advice - do plenty of questions, as this will get you familiar with the exam technique and help you master definitions for physics. For maths, try and find difficult questions once you've got a solid understanding of the basics, but make sure you have understood the basics first or there's no point.
Good luck!