Now with a benchmark for my 100s (and 400s) from the CSS test last week, I got back in the pool for an endurance day. That means that you just swim 300s, consistently.
As I had mentioned before about going out too fast, the first 25 or 50 of each 300, I was right on course. But for the rest of the 300, I was behind because I had spent too much energy. Something still to work on (among many others).