There are stream online of people doing mental work for 10 hours (with 5 min bathroom/snack breaks). I don't know if it's genetics or training, but some people can do it.
Cal Newport's "Deep Work" is another approach to this, turning maximum productivity into a daily, scheduled system. I've also been influenced by "Four Disciplines of Execution", specifically the first discipline - pick no more than three Wildly Important Goals, and do not work on anything else. If it's not a Wildly Important Goal (and it's not an actual emergency), it deserves zero attention.
I've been lucky at my current gig to be able to speak this language, to talk about the cost of interruption. When you have one job, you can focus 100%. When you have two, you don't get 50/50, but more like 40/40, with the remaining 20 lost to context switching. And the more often you context switch (or "multitask", hahaha), the more focus and momentum you lose. I'm sure a lot of knowledge workers lose north of 50% of their available productivity to the thermal losses of context switching.
My opinion of course :)