I've realised that in my breathing - in the pool, not the mat (not been there for over a week) - I've not been following best advice when doing crawl. The best method is to breath in at either side, every 2 3 or 4 strokes, and then breath out constantly until breathing in again. I, on the other hand, have been holding my breath. I breath every three, and breathing out for all three was really difficult to start with.
Chatting to a friend in a masters club, I learnt about the following drill. Pick a distance, whatever suits you to do a few repetitions without having to stop for breath or for others. I did 25 metres, as that's as much as I could do in my busy pool before waiting for someone slower to get ahead, or pausing for someone faster to overtake. In the masters club, they do 100 metres each time.
Do that distance breathing every two strokes, and doing the constant out breath in between. Then the same distance breathing every three strokes. Then every four, then every five. In the club, they go up to seven, but for me it was five that took a big effort to keep that out breath controlled and constant - constant enough to avoid that gasping-contracting-completely-empty-for-a-heart-beat feeling.
Then after five, it's four, then three, then two. And on this second stage, as the number of breaths goes down, suddenly the 'every three' that I'd been concentrating so hard on became easy peasy.
Every two breaths, on the other hand, makes me feel dizzy.
Got my distance up to 1,250 (50 lengths of my short pool) and it felt great. I'm not sure why under 30min swimming on an empty stomach is so much harder than over 2 hour yoga on an empty stomach. When my distance gets up further I'm going to have to think about a snack before I leave.