One of the most important skills in poker is knowing when to quit, if you become tired after 1 hour then quit the moment you realize you're starting to play bad. Don't keep playing on while playing/feeling bad, it really isn't worth it.

As for increasing the amount of time spent playing it's all about small consistent steps.

"I made a post once in HSNL about a thought which helped me a lot. It basically was the idea that every time the action is on you, it’s an opportunity for you to make the perfect play. Thinking about poker that way is good for your game."

Phil Galfond