I would follow the advice to use a softer reed until your endurance builds up.
A harder reed will tire your lips and cheeks quicker. The muscles surround the mouth need training, just like a weight lifter does training. With constant practise, your lip and cheek muscles get toned up, stronger, giving you greater duration.
Back in the early 1970's stationed at the U.S. Naval School of Music, I was playing my clarinet about 8 hours a day. I had 3 one-hour periods of concert band, and hour of drill band after school, and was also on a mandatory 10 hours of practise a week. (One checked out a practise room, length spent was recorded by one of their employees.)
Initially, I tired out quickly, had to take many short breaks. When I completed 6 months of school, I could blow my clarinet for 2 hours straight without tiring.
As long as you continue to practise regularly, your embouchure will strengthen and so will your endurance.