Hi Pete,
I feel I know what you're talking about, and think that many here above already gave you some good insights. However I want to highlight Wolftalker's point on O2 exchange improvement.
Like stated by Rik your VO2 max goes up with running. Meaning your muscles are getting better in extracting O2 from your blood. But does a high CO2 level seems to improve this ability? - I don't know yet.
A model.
Trying to envision a mechanical model for this, I like to think with Aerobic training, your blood vessels and muscle cell membranes get better in 02 throughput, 02 bandwidth.
Now when you consider the distance the O2 needs to go into the muscle tissue, you'll see there is a "bandwidth" which depends not only on throughput but also distance to the 02 rich blood. When we sprint our muscles demand more 02 than they can get, thus forcing the muscle tissue to start operating anaerobe. With running this threshold value will rise allowing you to perform in an extended aerobic range. This range could be 0 - 10 Mph for instance, above that your muscle will shift to anaerobe mode.
Changing your O2 bandwidth.
To get your body back to a lower transition speed, you can either stop with aerobic improvements, or increase the needed volume of O2. Meaning that your muscles will again have a greater demand than the bandwidth allows for.
Knowing that a good aerobic ability is beneficial for longer session time, and better cardioid performance, the focus of my training is the latter option, increasing the need for O2 bandwidth.
Increasing the need for O2 bandwidth.
In dynamic training I make sure that just before I get contractions I step on the gas and swim faster than my body is able to supply O2 to the needy muscles. This will ensure that the O2 in my blood will be available to my brain, heart and lungs, and not (so much) being consumed by my muscles. I've tested this on land and in water, yielding two personal best, after years of stagnation.
The other thing you can do is build more muscle mass. This will extend the muscle to vein distance and thus lowers the O2 bandwidth too. At the same time it increases blood volume, and carry more O2. About 50% of the O2 we have is in the blood.
Model verification.
Rik, and others, I've this model distilled from experience and observation, trying to understand the mechanics and principles, can you please verify the valid and invalid for me?
Mental part.
Apart from the mechanics, there for sure is a mental component. I think your enjoyment of freediving is not solely reliant on numbers though. Changing the learning focus to different aspects will allow recovery, understanding and growth.
Keep on asking, for you questions are teaching me greatly
Thanks,
Love, Courage and Water,
Kars