When you want to increase the range of the workout thats when you start messing with your RPMs, and gears and what not. This is where it's no longer just normal exercise and cycling training.
For a good cardio, this is where you are going to work in some sprinting. You are going to have to learn how to work your gears on this cause I can run through three gears on a sprint. This is one of the points where you are going to be exceeding 100RPMs; you’ll probably hit around 120.
You can also try climbing hills without downshifting. You want to get to the point where you are obviously expending way more energy trying to cycle up a hill then you would if you were walking.
Other things you can consider doing is Max V02 training and LT training.
I have gotten better results with eating fruit for long rides.
I remember after my first week of cycling I lost a healthy three pounds.