It is very commendable that you should wish to enter the VCE Latin course in year 12 after casual study in previous years! The main question is this - how casual was your casual study? It is important that you go into the Year 12 course with a very thorough understanding of grammar, and continue to revise that grammar throughout the year. It is perhaps worth going through some of the exam unseens from the last few years and seeing how well you can get through them in 30 minutes or so.
The unseen is the most important part of the exam because in recent years it has been what separates students. You will be able to prepare very well for the Virgil section of the exam, provided you begin your revision work as early as possible (in your case) and are able to have a good teacher with whom to work through scansion, language techniques, and the ideas in Virgil.
At my school, our teacher gave us lots of commentaries on Virgil. While these were interesting, I don't think they were necessary to get a high score. In fact, they were probably a waste of time/distraction for some of my friends who actually compiled and memorised all these notes on language techniques. It saved a lot of time to just rely on Literature analysis and English language analysis skills and to work with them, and it also means you are answering the questions using your understanding and not your memory.
I think it is great that you want to pick up so fascinating a language as Latin. I picked up French in Term 3 of Year 11 and ended up doing fine (43 scaled), and had a lot of fun in the process. It all depends on how hard you are willing to work!