Open Water Swim in Oct.

I'd personally amp up your weekly yardage, but do so by swimming longer rather than more frequently (e.g., if you're doing five 4K workouts a week, go for five 6K workouts or four 7k to 8k workouts) and having your sets be longer. If I may quibble, I'd recommend approaching it sort of like training to run a marathon. One long workout or steady swim at a long-distance pace per week, approaching and maybe exceeding race distance. Two other medium workouts and two shorter workouts, one of each at a higher intensity than the other. Three weeks hard, with the long swim increasing each week; then one week lighter. So to build from your current program over three months you might start the first month by working in one day per week where your workouts get longer by 1000 or 1500 per session, with the last week a lighter rest-ish week. Second month increase the long day from your prior month, and bump up two other days too. Third month same pattern as the last hard week of the second, and do your last pre-race long swim about 10-14 days before the race itself. Do be sure to get some OW experience to feel comfortable navigating, and also to be sure you can handle as much time as the race will require in water at the temperature the race venue will be. Also consider complementary drylands, especially shoulder work to avoid injury when you ramp it up and work to strengthen the low back. 2-4 hours swimming without a flip turn can give you a backache if you are not strong in the lower abs and spinal extensors.

I'd personally amp up your weekly yardage, but do so by swimming longer rather than more frequently (e.g., if you're doing five 4K workouts a week, go for five 6K workouts or four 7k to 8k workouts) and having your sets be longer. If I may quibble, I'd recommend approaching it sort of like training to run a marathon. One long workout or steady swim at a long-distance pace per week, approaching and maybe exceeding race distance. Two other medium workouts and two shorter workouts, one of each at a higher intensity than the other. Three weeks hard, with the long swim increasing each week; then one week lighter. So to build from your current program over three months you might start the first month by working in one day per week where your workouts get longer by 1000 or 1500 per session, with the last week a lighter rest-ish week. Second month increase the long day from your prior month, and bump up two other days too. Third month same pattern as the last hard week of the second, and do your last pre-race long swim about 10-14 days before the race itself. Do be sure to get some OW experience to feel comfortable navigating, and also to be sure you can handle as much time as the race will require in water at the temperature the race venue will be. Also consider complementary drylands, especially shoulder work to avoid injury when you ramp it up and work to strengthen the low back. 2-4 hours swimming without a flip turn can give you a backache if you are not strong in the lower abs and spinal extensors.