I give credit to a Lifeguard named Kacey at SimonFraserUniveristy in 2001 who coached the Masters Swim program for putting me through some of the nastiest challenging pool workouts of my life. I don't really like doing lots of distance swimming anymore just short sprints, but another good breathe control drill is pyramid breathing freestyle stroke. Breath on 3 strokes, then 5, then 7, then 9, then 11, then 9, then 7, then 5, then 3. That should be about 4 laps or 100m usually 2 sets done as a cool down exercise.