My alveoli are burning

I'd like to note some further unusual bodily activity that manifested during my initial introductions to running. I'm delighted to say the pains in my chest were not a precursor to an imminent heart attack. I'm far too fit for that (touch wood). It was my Alveoli trying to expand ever further for increased oxygen gathering that caused this sensation in the lung.

I write the above in past tense, because since dropping the speed down from 9.6 to 9.3 it's made the world of difference. I ran for 20 minutes tonight, with a 2 minute pause between 10 minute bursts, and everything felt great. Breathing was normal. Legs were normal. Everything was normal. In fact, I am becoming increasingly suspicious that the running could actually become enjoyable.

The above session was followed swiftly by an hour of swimming club, which again passed without any major demands placed on the lungs. I wonder what I might have to do to exert myself fully?

Current weight is still 16 stones and 6 pounds (230 pounds / 104.5 kg)