AHs are indeed, an excellent way to prepare for uni because they are obviously more difficult but the emphasis is on getting things done yourself. My classes (all Arts subjects - it may have been different in Sciences) were more like lectures - we sat, we listened, we took notes and any work was done outwith the time allocated for the class.
This is why you should never say "so much study time"! I thought 14 free periods (for three AHs) was absolutely fantastic. In June, when the timetables changed I was doing lots of reading outwith the set texts. When we came back to school after the summer, I studied solidly in every free period and for an hour or two in the evening as well.
Come April, just after my French oral and before the dates the dissertations had to be handed in as well as my creative writing folio, I was studying hard in every free period, as soon as I got home from school right up until I went to bed - which hit the early hours of the morning at its worst.
The good thing is, hopefully it will have paid off in that the coursework that was handed in before my exams counted a fair percentage towards the overall mark. But when I was cursing having to cut around 2,000 words of my English dissertation, it didn't seem too bright!