Running

This is interesting to me, probably hugely boring to other people, so apologies in advance:

When running I can easily push beyond the bounds of my cardio system, simply by running faster and faster, or finding a hill I can push my HR to 180+ and myself into the anaerobic zone, which means I have to slow down.

On the bike I've never achieved this- my legs fatigue first, whereas whilst running I could, if the CV system supported it, keep going faster and faster.

Is this what everyone else experiences, or is my CV system dreadful?

Running is far less effiecient, and uses loads more muscles. Your cycling glutes and thighs could'nt use the sort of oxygen load, your running body uses. Its holding you upright for a start.

Try cycling while doing little press-ups on the bars, pointing heel then toe, and bouncing your bum up and down. Once you start engaging more muscles. Maybe you'll be able to process more oxygen, and require a higher HR.