Just ride. As your aerobic capacity increases, your ability to maintain the higher cadence will also increase.

I agree with the advice you have been given about putting in time at moderate to low HR. That does not mean you can't go hard form time to time, or enjoy attacking some hills, but to develop your aerobic base there is nothing to beat long, steady rides. Once you've ridden 1000 miles, come back and we'll talk about more specific training.