To see a video of this breathing technique, click on the link at the bottom of the article.
I've been racing the 1,650-yard freestyle (the "metric mile" in a 25-yard pool) for almost 40 years. It's never been easy, but in middle age it's gotten considerably tougher. The problem isn't the distance; it's all those flip turns!
If you turn the way coaches say you should, it means going without air for a significant portion of the race. You usually stroke one or more times without breathing just before the turn—the better to maximize your momentum going in—then try to hold your pushoff after it because you're faster while streamlined under than stroking on top. This can interrupt your breathing for four to five seconds every length.
Even in my teens, the cumulative effect of such interruptions in a 65-turn race meant that in the final lengths I felt I needed the whole length to get my breath back; and just as I did, there was the wall again. You can imagine how much more taxing that feels at 57.
One little trick I've learned in mid-life is to sneak in one more breath immediately before my somersault in the middle and latter stages of a long race in a short pool. If my last stroke is on the left, I take a right breath immediately followed by a left breath going into the turn.
The slight delay in the onset of oxygen deprivation and bit of extra O2 to hold me through the pushoff makes a world of difference: I'm more comfortable and the extra oxygen going to my brain even seems to aid the concentration it takes to hold my stroke efficiency as fatigue mounts.
It does take practice, though; when else do we practice breathing on consecutive armstrokes? And then you have the wall in your face as you complete the second breath while trying to whip smartly through a somersault. For the uninitiated it can turn a smooth maneuver into a funhouse tumble.
One way to practice is in mid-pool. Swim several strokes, breathing on the last two and then immediately somersault. Practice both a right-left and left-right breathing sequence since you won't always come to the wall on the same armstroke.
When you can do breath-breath-somersault without feeling discombobulated in mid-pool, move your practice—cautiously—to the wall. After that, there's no substitute for doing it regularly in training. During virtually every long swim or set in practice, I begin with regular pre-turn breathing, then progress to consecutive-breathing as I need more air.
You can watch a quick video of both types of turns here.