This thread kinda rubs me the wrong way. I remember after taking my ACT in high school I immediately cracked open Kaplan MCAT prep books and started studying. I also started taking free online courses covering pre-med content, in order to make sure I was learning things properly and not merely memorizing a set of notes.
I'd say this helped me maintain a near perfect GPA at a T20 school, while pursuing an extremely rigorous course of study (the kind most pre-meds shirk away from). It also taught me how to use Anki effectively and gave me tons of time to do research that led to publication in undergrad. It also gave me a good MCAT score without having to "reteach" myself concepts.
For STEP 1, I'm sure medical school will go so much more in depth, like everyone here claims. Either way, having an extremely strong intuitive understanding of physiology and other subjects will only put you at a competitive advantage compared to someone who is literally seeing the content for the first time. Anyone who says otherwise is being disingenuous.
So yeah OP, I think there's nothing wrong with pre-studying, especially if you don't go crazy doing it. I still manage to go to the gym and watch netflix every day. Just make sure your personal relationships don't falter (something I'm working on) and make sure to develop a life outside of medicine too.
But don't stop learning if that's what you love to do. Personally, I want to go into academic medicine, so I've been spending this last year trying to churn out some more papers. In my downtime at lab, after I've finished classwork, I throw on a Najeeb lecture. There's only 700 or so, so watching several a day at 2x speed while making notes with a prep book is doable. And no one says this doesn't have to be fun. I watch the videos on my favorite organ systems first.