Don't worry the initial interview is just a chat about careers options within the Navy, nothing more. It's a chance for you to ask any questions you may have about the process for getting in or life within the RN.
You should fill it out in pen and take it with you when you join HMS Raleigh. It is supposed to be the last 8 weeks before joining. You will be given a copy at your pre-entry brief and this is the one you need to complete and take with you. Lets be honest I'm sure most people make it up, but they are only cheating themselves, because if they can not do the fitness it will be found out during your initial training at HMS Raleigh.