Denis, I wrote this as part of a longer piece:
"100 I focus on breathing out. I have to fight my natural instinct to hold the air inside my lungs until my face is back out of the water. I held air in my lungs all through high school, through masters, through triathlons and up until about 1999. I signed up for a local YMCA class on competitive swimming tips. The class was for kids, but they let me in by mistake. The first thing they taught was to put your head under the water and blow bubbles. Holding air has been my most difficult bad swimming habit to break.
150 Your lungs absorb the oxygen quickly, so there's no reason to hold dead air in your lungs. Holding that air needlessly stresses the muscles surrounding your lungs, and tires them. When I used to swim longer than 250 yards, a voice inside me used to start inventing reasons to stop at the next wall. There were plausible reasons, "Your goggles are fogging/leaking," and very persistent ones, "You really need to stop, Now!" I'd argue back, "Look, I did 1000 two days ago - there's no reason I can't do it again." Sometimes I won - I built up to 3000 yards while training for a triathlon - and sometimes my lungs won - I would suddenly decide there was no room for a flip turn and stop. A few years ago the voice stopped, but I have to be vigilant about exhaling."
Terry has added breathing advice since I took my "swiminar" in the late 90s, but back then taking fewer strokes per length brought my bad breathing habits into sharp focus, and it took a very long time - years - to get past them.