I've ridden flat bar bikes for 4 years & many thousands of kls including 4 & 5 hour rides. Often a bike with to long a top tube can also put to much weight on the hands as well as a forward seat as previously mentioned.
When you do get the weight on your handlebars correct, try some of these tips:
Don't:
a) Sink your wrists, have your hand almost in the same plane as your forearm
b) Grip to tightly, a soft grip also lessons fatigue of the arms
c) Pump the front tyre to max pressure, particularly over bumpy surfaces
Vary your hand position often, & yes get bar ends. These are very useful when pedalling standing, which you should do often anyway, even for 30 or 40 metres[further as you become more accomplished]. This varies the muscles you use, again lessoning fatigue, not to mention giving your bum a rest
I ride flat handed a fair bit, even over rough/bumpy surfaces [where the natural reaction is to grip the bars for fear of falling ] with my fingers resting on the brake levers.
Some of these take practice, so like all new skills try them when you’re not in traffic etc. Eventually they will become second nature.