Is it really impossible to “grind” all day?

Every time I seem try to work straight all day I end up losing the ability to work after a few hours of work. I remember reading in Cal Newports “Deep Work” book that you can only focus purely for 3 hours a day on a task. Yet, I’ve heard in music and pop culture that you can grind all day if you want it bad enough.

I’m wondering if anyone is actually able to do pure work, especially in a creative space like YouTube editing or learning something from university work?