My endurance is pretty lousy, and has been since I was a child. I overheat quickly, and it's not hard for me to get out of breath (it's improving a bit, with me doing running more often). One of the things I started doing several years ago, was forcing myself to not hyper-ventilate when out of breath, but rather keep a steady rhythm, and try to take in and put out more air. Occasionally, this has backfired, with me finding that I need to stop exercising for a bit because I'm greying out from not getting enough oxygen, but usually it's sufficient to breathe steadily during exercise even if I continue to feel like I'm out of breath.
I've read a few articles on competitive breath-holding where they state that the practice of doing it sometimes involves less the ability to get by with less oxygen and more teaching your brain to ignore the warning signs of oxygen deprivation, leading to problems like shallow water black-outs because you feel fine right up until you pass out. Obviously, above water, this is a bit less dangerous (although I do generally try to avoid passing out, and instead cease the exercise, or let myself gulp breath if it comes down to that). That said, is there any danger that I am, like the underwater breath-holders, masking the symptoms instead of improving my condition by forcing myself to breathe slowly?