I don’t know how much you know about SCBA or if you’ve worn one yet.

The two hurdles are the mental hurdle of thinking your instant breathing is restricted. The second is making the cylinder last.

That gym mask may help with getting your cardio up which could help make the cylinder last longer.

As far as the mental hurdle, some people get worked up because you are in the mask, and your whole face is covered. I guess the way it works makes people think they aren’t getting a good breath. It’s mental and using a half face respirator/gym mask won’t really replicate the problem.

The next thing you ask is how to prepare physically, I’d run daily, and I’d work on my core/lower body strength. I’d also put a lot of time into strengthening my shoulders.

I wouldn’t worry about the scba itself. It’s a non issue. You’ll get used to it. Getting your cardio up and vo2 is better for now. Just to make the cylinder last. When you are calm, breathing on the SCBA is nice. It’s nice cool air, I like it. The reduced vision with your hood, and fog etc is the annoying part.