That sounds suspiciously like you might not be a mark 5 drawing robot, but a human being! I would let that check out!
Jokes aside, 2 hours focused concentration on a task IS exhausting. Most people wouldn't even get the idea to aim for training to draw more than that at once. I personally rarely rarely have a day, where I manage to draw 3 or 4 hours, but those are especially inspired days, and then its generally not in a single setting.
I know that professional artists and some enthusiasts manage to draw 10+ hours at once. But I think getting there is less a problem of strictly developing drawing skills, it's more a question of getting used to regenerate efficiently when taking breaks. Or even using mentally less taxing parts of drawing, like filling in large shapes, for regeneration.