I have started running again and in 2 weeks I have noticably lost weight and I think I have improved my fitness to a better level.
The best way to get fit is running (in my opinion)- remember that it is not how far/long you run for, it is how hard you run. Consistancy is also a key factor.
You should not have to resit prelims, the HSC course if completely different and it is actually good to fail them because you know what not to do in the actual HSC.
I do not have a job like you but you should try to simplify your life as much as possible in order to prepare for the HSC. My afternoons generally consist of 1 hour of study, excercise, relaxation and then most importantly sleep.
How many hours are you sleeping each night?
Re: study time
I have already been making summaries. With only doing an hour a night I am up to date with everything. Hopefully I can continue this throughout the entire HSC year.