I am current doing AS further maths, and planning to continue it next year.
The work load is manageable(as long as understand the concepts), roughly, I have always been amongst the top 10 mathematician in my year, and I am doing it along side with 3 other subjects.
As soon as you finish C3,4, you would find C1,2 very easy. S1 is hard as first, but practising is of course the key. M1 is the one that I struggle with the most.
I think the best way to get through it, is to be genuinely be interested and love maths. I absolutely love my course up to now. Though, I think i would get a tutor during summer to be more prepared for next year's course. I have friends who are very smart, smarter than me, she manages it, yet absolutely hates it. Trust me, you don't want to be her!