(This is from what I experienced last year, having done Edexcel Maths at GCSE and then Edexcel Maths AS Level)
C1 has essentially very similar content but less of it, in that there is not much data handling etc (which is covered in any Statistics modules you may do) or the more Physics-related work (which is covered in any Mechanics modules you may do). Since there's less of it, you tend to go into a bit more depth and do more practice for the work that is included.
The final two chapters of the book are integration and differentiation (from that I remember?) and whilst that is new material to learn, it's not particularly difficult.
Essentially, C1 is there to make sure you have a good foundation for the following Core modules.
Just because it's going well so far doesn't mean much, to be quite honest! Last year, I got an A in C1 but a C in C2...