I would agree that you will likely have to build up to it. I also wonder whether you also need to work on your pedal stroke. Out of curiosity, did you happen to note what gear you were using when you tried to sustain 80 rpms? Next time you go out, try to note what gear you're in and let us know. I have to wonder whether you're still trying to push too big of a gear, but in saying that, I will admit that I didn't struggle with this as a new rider. So perhaps I'm underestimating how challenging it might be to someone.