Assuming a fairly constant speed and level terrain (hurrah for canal-side paths!) I can poke along at 70rpm for miles, can sort of maintain 80 if I think about it, and can inch up to 100 for a short while (I have plain old pedals). Then I start huffing and puffing like an out of shape middleaged woman. Uh, wait ...
Sounds reasonable to me.
So I know that I want to increase my aerobic capacity and get a higher rpm.
Why? What are you trying to accomplish? What is your ultimate goal? RPM is not really a goal, though it might be a means to a goal.