You are right about stamina being important. It's one of the qualities pros value and you may have to train for it. Playing blitz does not train you to be able to concentrate intensely for long periods of time by its very nature. To train for long games you have to play long games. Say start with moderately long games like 15 min + 5 30 sec. byoyomi periods and move on up into the 30 min + 5x30sec. byoyomi or even 60 min + 5x 60sec byoyomi range. Eventually you'll be ready for 6 round tournaments played over two days.