Your reactions are quite normal.
The high breathing rate is mostly caused by fatigue of the respiratory muscles. Here is an interesting paper:
“Our results suggest that expiratory pulmonary capacity and lower-limb strength are more fatigued following a longer MUM, while inspiratory strength loss is independent of distance.”
You can train your respiratory muscles with special devices. I had good results with that. It’s cheap and not much risk, so I would recommend you try that.
breathe like your life would depend on it!