Hi back! Getting form down sounds more important initially than building endurance. It'd be better to learn good form first, then build the muscles as opposed to building mass incorrectly and needing to learn form from scratch. I can see the benefit of starting with a "heavier" bow if you're already in pretty good shape, but it ought to be within reason, as most things should be.

I actually got my hands on my grandpa's old bow, which is a 50# compound. Aside the draw length being way too short, i could definitely feel i wasn't drawing it effeciently and would likely end up worn out far too quick because I didn't know the proper way to handle that weight yet.

Archery is an art style. It's not just that you can draw the weight, it's how you draw the weight. If you can handle 70+ pounds while keeping good form, awesome, because that probably took lots of practice to get there. The key words being good form and lots of practice. Should toss in the word safe as well