I'm doing both maths and further maths (edexcel) and am currently in my AS year.
AS Maths is all right I guess. C1 is the A*-B topics from GCSE with a few expansions into Sequences and Series, Differentiation and Integration. It's nothing difficult, if you're good at GCSE maths and are achieving A's and A*'s consistently, you will find C1 very easy (or at least I do).
C2 is more difficult than C1 but isn't too demanding, there are expansions into more new topics such as Algebraic division, remainder and factor theorem, more differentiation and integration, binomial expansion, more sequences and series and lots of trig (nothing too difficult, a little more from GCSE and intro to trig functions and graphs). To be honest if you are good at maths in general and enjoy it you won't find C2 too be much of a challenge either. Just make sure you do C2 questions frequently, make sure that you also don't fall behind and you should be ok.
S1 is the most boring thing I have ever seen, it's all right as well, a bit of an expansion from GCSE but not too much of a step-up, BUT IT's SO BORING AND TEDIOUS, every time i am in an S1 lessons i want to smash my head through a wall. But in terms of difficulty isn't too demanding either.
Now for further maths(the FP1 module), if you are good at maths in general you will also be good at further maths. Before you start it, it does look quite scary at first but isn't really all the hype (for me at least), however you do need to be good with your algebra. It is more difficult than C1 and maybe is on par with C2 in terms of difficulty. You will use a few C1 and C2 methods in FP1 topics, such as coordinate geometry, differentiation, remainder and factor theorem and sequences and series. As a result doing FP1 should strengthen your normal maths in those topics. In terms of how hard you must work, just make sure you don't fall behind and frequently do a few extra questions (same as C2).
For the M1 (mechanics)module in AS further maths, for me is the biggie. The ways of doing maths in M1 is very different to all other non-mechanics modules. If watch a few exam solutions M1 videos you should see what I mean. I took me about 2 months to get my head around it (didn't help that I had a s*** teacher). Once you understand it, again just do a few extra questions per week.
D1, decision maths, isn't even maths tbh. It's all using algorithms, using different methods to solve different problems. To understand what I mean, try to look at a D1 textbook. It is very very different to conventional maths, but I think is the easiest out of any maths modules. I find that there is no thinking in it at all, and that when working at it, it's all feels quite automatic.
On the whole nothing too much to fear, just make sure that you achieve A's soon A*'s consistently at GCSE, then once you reach AS-Level just make sure you work, do some questions and read ahead a bit and you should be ok. I haven't gone into A2, but from what I have seen form myself it looks like a different planet compared to AS.