Can't speak for other trades.
The job of an Infantry instructor is to break you down and bring you back up to the school's standard. When you get to battalion, to a degree, this process will begin again as you learn that particular battalion's SOPs.
Running for moderate fitness is fine. Being in decent condition is great. But the most important part is shutting up and learning. Leave ALL your misconcptions at home. Basically, you didn't know anything before you got where you're going.
Some people will disagree with this. I'd suggest ALL of them have no idea what you are about to get yourself into. The instructor cadre are all experienced at their jobs and are they are the masters of their world. They do not care what your opinion is on the importance of Karate training or the best rifle system in the world. They do not want to know your political views. They expect you to absorb their view; and it is a very utilitarian, CF centric outlook.
As for the notion of impressing someone with your answers? No. Trust me, you don't want to be getting anyone's attention.
Middle third dude. Learn how to be a grey man and not stick out. People who don't listen to that advice? Well, they'll stick right out. And a very select few of these are simply just exceptionally tough people that don't have an aversion to the side effects of being s**t-magnets.
But attitudes aside, these troops cant rise in rank as effectively because they are always being charged and demoted. I know one ex-3 Cdo guy now working an Atomic Energy security gig who was a Mcpl most of his career due to his propensity to violent tendencies. But he lives a hard life most normal people simply cannot hang with and is the exception that proves the rule. My very first bar trip out with this individual resulted in a brawl with a city league football team on principle that when in town you should go introduce yourself.
But he's exactly that guy, and in my continuing opinion, embodies exactly the level of aggressive fighting spirit an Infantry battalion should posses.
As for being fit to pad you from fitness culture shock. You could show up as an olympic hopeful distance runner. They will take that preconception and run you in pack and boots until you are exhausted as any other body. Because its not what you do when you're at your best that makes you who you are as a soldier.
Its defying your limits and rejecting the things that stop you.
You're making a noble career choice. Have a good one.