I do a set sometimes for training new TT thresholds.
set a comfortable SPL, probably 14 for you.
Then swim repeat 25s inside that SPL with decreasing TT. You have to swim 3 in a row at a TT before decreasing increment of ,02 or ,03.
Set yourself a time limit and see how far down you can go in that time.
e.g. start TT at 1.2 and try to reach 0.8 in 45 minutes. As the TT speeds get quicker, i find the body naturally gets more focused, my leg kick timing improves, my symmetry is better and my recovery quicker.
I can also keep precise focus because I know I only have to succeed the task over 25m *3
the 3 in a row is great focus training too, as you get really annoyed with yourself if you blow the 3rd length when you are holding 2 successes.
Lots of marathon swimmers seem to have SR in the range 60-80 so,
As stroke length conscious swimmers we may be in that lower range but I do not think the demands on the body of moving from SR 50 to SR 60-65 are as much as we think they are, just getting used to the new rhythm.
Thats going to be my goal for the next couple of weeks, SR 61 and hold SPL under 18 in perpetual motion.