One my favourite swim sets, actually my only favourite swim set, as it's the only one I've returned to many times, is this 40 ⨉ 50 set with pace changes.
- 4 ⨉ 4 ⨉ 50, 1st 50 fast, the rest steady, on 5 seconds rest
- 4 ⨉ 3 ⨉ 50, 1st 50 fast, the rest steady, on 10 seconds rest
- 4 ⨉ 2 ⨉ 50, 1st 50 fast, the 2nd steady, on 15 seconds rest
- 4 ⨉ 1 ⨉ 50, fast, on 20 seconds rest
It's a set I learned from the Endurance Corner swim library, scroll down to the “EC-S9_PC 50s Swim” heading.
I did this one today, except I'm getting old and my memory is failing, so I had 10 second rests all the way. He who does not remember must train harder. And I fail to see that as a bad thing.
With a 400 warm-up and a 200 m cool down it's a great little 2600 m session, and I did it in 51 minutes today.
Seriously effective, because you get a total of 800 m of fast swimming, and you get to enforce the habit of progressive swimming, it automatically and gradually gets harder and harder throughout.