Daleport996 said:
How do people draw really fast? And I don't mean "practice", I mean how do they draw for 3 hours straight without taking breaks or getting distracted for twice that time?

From my observation of artists, the answer is 'obsession' :)

But if you just want to develop your ability to draw for longer, going through an actual curriculum that required me to draw for different periods and in different ways helped me. The author of the curriculum I used (Nicolaides) does explicitly warn that longer drawing is something that must be built up to, and trying to just go straight to it is liable to stupefy you and burn you out.