I have my first Specialist SAC as well tomorrow. Doing it by DECV so I have no real idea what to expect (i.e. not like I've been given practice SACS or hints by a teacher in class). Not going to be fun, pretty sure I'm going to get destroyed.
I can tell you what the DECV sacs were like last year. For all I know it might be completely different style this year, but anyway:
The first few are multi-part and over a few days. I remember the first DECV sac I did being quite hard. It felt like the hardest thing I'd done in high school up until that point. Not that it actually was, but yeah it was more challenging than I was expecting. So yeah, that first session was hard and I felt like I was going to fail.
Overall it was kind of fun and they ask good questions that require a bit of time to think over. Once you figure out the answer to the question is the best feeling.
Break the questions down into parts, so that they're easier to chew.
It was a set of less than 10 (not that many), pretty long questions. Since they're multi-part so you can make use of the time you have outside class. You can go home and think over the questions (if you can manage to remember them, usually just remembering what general concepts you needed to read up on was easy enough to do), figure out an approach etc.
I honestly don't think I would have done as well as I did on the DECV spesh sacs if it weren't for the time I had in between to read up on concepts and ponder questions.
In your DECV booklet thingo, there should be the marking criteria in there. Read it, and read it closely. You want to show all working, talk through the question etc. on paper because that somehow or other fit into the marking criteria.
aw that sucks 
Just curious, how do you get ranked if you do a subject by distance?
DECV is basically another school, so it works the same just as any other class in any other school. They don't tell you what you're ranking was for sacs though, they just tell you if you were in some kind of grade block (Very High, High, etc.)