Here is a progressing series of sets I use to train for long distance open water racing.
Week 1 –
Day 1 – 4 x 500 first 100 fast, 50 easy, balance at race pace 3 min rest between reps
Day 3 – 5 x 500 first 100 fast, 50 easy, balance at race pace 3 min rest between reps
Day 5 – 6 x 500 first 100 fast, 50 easy, balance race pace 3 min rest between reps.
Each week I start back at 4 reps but increase the distance by 100 total per rep and increase the fast part by 50 per rep. so Week 2, Day 1 would be 4×600, 150 fast, 50 easy, balance at race pace Week 3 Day 1 would be 4×700, first 200 fast, 50 easy, balance race pace. and so on. I do this for a 5 week cycle. It is important to focus on going really fast for the first part of each rep and returning back to race pace for the balance of each rep. The goal of these sets is to improve ability to swim long distances at a fast pace and to get used to going out fast. In open water swimming this will help you to get out with the front pack and free from the crowds. As you can see I do this set three times a week. On days 2,4 and 6 each week I do sprint and technique work.