The distance swimmers did a variation of this main set once or twice a week for a whole season. I swim this a lot during training for triathlons now. What is great about the main set is that you can modify the length to fit what you need to do.
A note on intervals, everybody has a preference with intervals. Personally, I love doing sets where I have an alloted time to complete each swim in and will get my rest based off how fast I go. I make my intervals challenging because that is the best way to get my heart rate up. I’ve found a lot of people who didn’t grow up swimming prefer to have a fixed amount of rest. Whatever you prefer…do it. When your swim workouts are more “enjoyable” to you then you will be more likely to push yourself and get motivated to swim fast. Let me know if you have a question about finding out which interval works best for you…it will vary vastly from person to person.
Warm up: 2 x 200′s easy @ 15 sec rest
Drill Set: Repeat 2 times – 3 x 50′s Drill #1 @ 10 sec rest, 1 x 150 perfect stroke @ 20 sec rest. Focus on one or two parts of your stroke for this set. For example focus the entire time on making sure your head is in the proper position or your hands are finishing all the way through.
Main Set: Repeat 6 times -2 x 100′s race pace @ 5 sec rest, 1 x 100 easy @ 10 sec rest. Your race pace is your goal speed for the distance you are training. Example: Distance = 1/4 mile swim (around 400 yards), Goal time = 6:00, Race pace = 1:30.
Cool Down: Easy 200 yd stretching out your stroke.
Total Yardage: 3000 yards.
Variations: Note for the main set you can vary how many times this set is repeated to make your workout longer or shorter. We did this set 10 times through. Also, you can do the main set on an interval such as 3 x 100′s @ 1:30 (2 fast, 1 easy) repeated 5 times. Another variation to get more yardage is to repeat the drill set more than twice.