Following many years of computer use and not going outside, I decided to try some exercise again. There is a swimming pool a few minutes from my house, so that seemed like a good way to go. Hadn't really swam properly for 10+ years, and first few times I went I found myself completely exhausted after just a few laps. Was trying to do freestyle, and had forgotten how to breathe properly, how to kick and all sorts of basics. Eventually I remembered to google some advice after getting home (instead of getting distracted by code). Three super useful things took me from "can barely breathe after 3 laps" to swimming patiently and properly enough that I now usually swim until I get bored or run out of time.
1. Mouth inhale, nose exhale
Initially I was breathing out through my mouth. This meant when I turned my head out of the water I had a brief exhale before breathing in. This eats into a lot of head-out-of-water time, and you need that! This great read suggested some out-of-water breathing to get used to it, then some practice in water until it feels more natural. Didn't take very long to get used to this again, and now have much more time to breathe while head is out of water.
Another one from that same link is to keep your head up while breathing (stay "tall"). I took this as looking forward when you turn your head, rather than backward. This keeps airway open and makes breathing much easier. Another obvious one, but you don't think of this when you are exhausted and actually in the pool.
2. Swim slowly
Probably an obvious one, got this one from a random person at the pool. Rather than going for speed, just focus on doing things correctly. Part of this is getting comfortable with swimming slowly and continuously. Goal here is to find a comfortable pace to just keep on swimming comfortably. This is helped a lot by:
3. Little kicks!
This made the biggest difference. Probably obvious, but my early attempts were doing big strong kicks to move a lot of water. Googling something along the lines of "swimming big kicks or little kicks" gave me a great article that I can no longer find. But the theme from other pieces is: make your kicks low effort. Small kicks reduce your drag, which makes you much less tired. And important to relax your legs - kick the way you walk. So move your leg from the hip, and just relax the rest of your leg. Too much effort from knees or trying to point toes will just make you needlessly tired. Kicking doesn't give you much propulsion anyway - apparently most of it comes from the arms.
There is still much improvement to be made, but this has been the first exercise I've committed to in a long time :)
Some links
https://www.triathlonmag.com.au/training/swim/7597-3-golden-rules-of-a-good-swim-kick-
http://www.enjoy-swimming.com/flutter-kick.html
http://www.swimsmooth.com/kick_adv.html