As others have said standing more will make standing easier. I can do it for a short time but I've never been one to stand for any distance. It's something some people can do easily and others, not so much.
I climbed a 9 mile canyon one time with a friend who stood the entire way. I rode across the valley here for about 10 miles with a local pro who stood the whole way. I was hammering along trying not to embarrass myself and he spun the pedals from the standing position, chatting, checking traffic, etc, like it was no big deal.
Reminded me of video I saw of Sean Kelly riding next to Greg LeMond and Sean's legs were spinning as he stood but his upper body was almost motionless.