I think the level of step-up from GCSE to AS somewhat depends on whether you did well at GCSE because you remember things well and study efficiently or more because you understand, are good at and enjoy your chosen subjects. While study skills are great, and necessary to succeed at AS, I think you also need to understand the subject a bit rather than be able to tick the mark scheme's boxes. If you're looking to do science/engineering, you probably enjoy your subjects and "get" them to a certain extent. In any case I also think the step up is blown out of proportion. Personally, although the work is technically "harder", I found it easier to cope with 4 subjects of overlapping, only slightly tougher material than 11 subjects of often unrelated stuff.
I believe hard work is more important than individual brilliance in any exam.
I did science/maths subjects and what I found to be the most important for me was making sure that I fully comprehended the subjects as my teacher went through them. After we finished a subject/bullet on the specification (e.g. "Standing Waves" in physics) I'd look back over the work and make sure there was nothing I didn't understand. This meant that when it came time to revise for my summer exams, I was actually revising as opposed to re-teaching myself parts of the course I'd never really paid attention to. That took a lot less time for me. I could go "Right, this. Okay. Got it. Next." (of course an oversimplification!)
If you make sure you don't fall behind and prepare yourself properly for your exams (as others said, you can't rely on just natural ability at AS, not because of the difficulty but because the nature of the questions means there's things you'll just have to know to get the marks, and for the "application of knowledge" bits you need to fully understand the basic principles) I think it's no large task to keep up the standard and get 80%+ for an A at AS. I'm not saying it's easy. I'm saying if your someone with mostly As/A*s at GCSE, then the level to which you're willing to apply yourself is enough to get the same at AS, provided you do it sensibly! Don't not revise for A-Level Maths! And remember it's only 3/4 subjects now so they'll obviously need more revision each...(some people apparently overlook this). Overall I only spent a bit more total time revising my AS than my GCSEs.
Take the advice of my personal Deity, Douglas Adams: Don't Panic. If you work hard, enjoy and understand your subjects (or at least be able to answer correctly the typical exam questions on them) and revise in plenty of time for your exams then you will do more than fine. You're not afraid of hard work and your post here proves you're conscious of what's coming, so along with your GCSE grades I'd say you could get AAA and beyond! Aim high.