I have never heard anyone say "Just do past papers". This is because it is not true; you need to do a lot more than just past papers in order to revise a subject. Past papers will give you practice doing question that test you on the subject; they will not teach you the subject. Most of your time should be spent going over your revision notes (provided that you have already made them; if not, us other people's notes) in order to revise the subject. Then, do past papers periodically, ramping the amount up as you get closer and closer to the exam.
If you start doing them now, you will run out in the period close to the exam - the time when you actually need them.