Been swimming a lot ever since I developed hernias and wait for surgery.
I think that a great workout is doing some HIIT -- here is what I have been doing, three days a week:
Day 1: 200-300m warm up. Then 5x50m on 5 minutes. All out sprints from the block. I do butterfly. It's rough -- I time myself and the other day I went 29s - 31s - 34s - 37s - 42s. Big decline at the end, I have that good early speed but need more sprint endurance. Then 200m cool down.
Day 2: same as above but freestyle. I have better endurance here and can get under 32 seconds for each one, first fast one is usually 27 seconds.
Day 3: 200m warm up, then 3x100 all out sprint Indivdual Medley, try to get under 1:15 for each. I am very slow at breast stroke so it's a bit of a struggle but I can get there usually. Do them on 7mins so I get about 5:45 rest. Then cool down.
Sort of a fun way to mix up a swimming workout, I find it excruciatingly boring to do a long 1000m plus steady state workout.