Question, or your open water swim are you in a wetsuit ?
You really need to swim 3 times a week, really make the effort to maintain the rest period no more, the goal is to get your heart pumping and your breathing under control.
(I know this program works, i recommended to it two colleagues at work, one was not such a good swimmer and was at week 4 for 2 weeks before moving on but that was more about fitness levels.
Good luck and stick with it, Swimming is really good for you
WEEK one:
4 x 100 meters ...rest for 12 breaths between 100s
4 x 50 meters...rest for 8 breaths between 50s
4 x 25 meters...rest for 4 breaths between 25s
total: 700 meters
WEEK two:
200 meters...rest for 12 breaths
4 x 100 meters...rest for 10 breaths between 100s
4 x 50 meters...rest for 6 breaths between 50saw
4 x 25 meters...rest for 4 breaths between 25s
total: 900 meters
WEEK three:
400 meters...rest for 12 breaths
200 meters...rest for 10 breaths
4 x 100 meters...rest for 8 breaths between 100s
4 x 50 meters...rest for 4 breaths between 50s
total: 1200ď meters
WEEK four:
600 meters...rest for 10 breaths
300 meters...rest for 8 breaths
4 x 100 meters...rest for 6 breaths between 100s
4 x 50 meters...rest for 4 breaths between 50s
total: 1500 meters