Try getting out of the saddle. Generally you are stronger in that position. It will take awhile for you to build up the strength to stand for long periods of time, but if you keep at it you will be stronger overall.

I have learned to gear up a bit before I stand as I don't want to spin standing- that's really tiring!!!!

Also, do you know what your gearing is? That would help us in this discussion.

Great that you have taken up cycling. It's a wonderful sport and great for your health.