It has been a long long time since I really trained for swimming. But one of the standards was sets of either 5 or 10 x 100 yeads going every 1:30 or 2 minutes. Each 100 you go all out and then get a little rest. Or doing similar stuff based on pulse. Sprint 100 yards, take your pulse and keep doing so. Once it is down to 150 beats poer minute go again. \
The point is going hard and getting just a little rest to recover lets you spend more time working hard and improves conditioning faster than just going 500 or 1000 yards.
Trading off between kicking and pulling sets was common. Different muscles, but overall conditioning continues.
One other thing was heading into meets to do 75 yards as hard as you can (this was back in age group where 100 was the standard distance). The idea was that everyone finishes hard, you see the end in sight and gut it out. But if you break things down by 25 year distances it is the thrid 25 that is both the slowest and the greatest difference between swimmers. I'm not sure this works well as a training technique for tris, but it is food for thought. Also people might want to train for a swimming distance longer than in hteir tri, esp for sprint tris. Difference is in a tri it is not over when you get out of the water and feeling like you are exiting the water with something left could be important.