the phys is all proggressive and builds up. they don't expect you to be able to do a 6 mile tab in the first week.
my basic started off with a few circuits and swimming once a week for the first four weeks as it's no impact so won't break you.
you then progress on to doing more green phys such as boot runs and tabs, with the weight remaining the same but the distance increasing evenly. the assault course is a lso a good milestone to reach at first.
you don't need to be super fit when you arrive, just be capable of showing off a bit in the first pft and running well. if your lagging at the back, your instructors will see you as weak from the start and just take more pride in trying to make or break you with some remedial pt.
just remember, when you here the words 'standby, go', you best start running, fast!