Getting the cadence right for any of us will. or would have, taken practice. Most newcomers try to get speed out of the bike on the flat by getting it into the highest gear and grinding away. Not the fastest way to ride, not the most energy efficient and definitely not good for the cardio vascular or the knees.
I would not like to say that there is a correct cadence on a bike as everyone is different, I have a natural cadence now of around 90 to 95, but can get a lot higher, and can still work at lower cadence. What I like to do is keep the same cadence for all parts of the ride. Plenty of gear changing going on, but to me the effort through the legs is not excessive. If it gets hard, I change down, and then down and down and when I run out of gears- I slow down.
The main thing is I try to get the gearing right for my leg to feel comfortable, and still keep MY correct cadence. I have a friend that runs a far lower cadence than me, and he is fast and Super fit. His cadence is around 60 to 65 and boy does he climb those hills.---- For the first part of the ride, then he seems to see sense and slow down. Still keeps cadence low though. If the ride is a little longer than normal, then we find the reason for his seeing sense as he is shatterred.
Way to train? Spinning classes help, but they can be difficult to find and may not suit everyone. Try to find a cadence that is comfortable. 80 is still quite slow for a performance rider but you may be natural below that.Get your cadence working at a comfortable level, and concentrate on keeping that same cadence for a 5 mile ride. a few miles at the same cadence will get yourself in the pattern of what your legs like. It will not matter if you are in a low gear at low speed, or higher gear at higher speed. just try and concentrate on keeping the cadence. Then take in slopes where you will have to change gear to keep the cadence. You may find that your natural cadence is too slow so you strain the legs on the hills. Try to raise the cadence a bit and see if it improves, or you may be spinning too fast and running out of lungs.
One thing I have learnt over the years though, and that is if it hurts--Slow down. If it hurts that much then stop.