RMAD (if you can attend one) is a very worth while session to go through. It will give you a very good indication of how you stand fitness wise and highlight any areas that need work prior to going to PRMC. It will also give you a rough idea of what is expected of you as well.
RMAD is made up of several tests from prmc - day 1 of prmc (3miler, 2mins press ups, sit ups & max pull ups) then something similar to gym test 2 on day 2 of PRMC.
There have been a lot of guys on the site who have attended, they should be able to give you more indepth idea of what else goes on.
RMAD is brilliant I think. You find out your best scores because your pushing yourself too the maximum. It also gives you the opportunity to try the warm ups they give you which you wont really be able to do yourself at home. Before I attended RMAD Id never even seen a Royal Marine before.