If you want to do a set to improve aerobic work levels without building up excess waste (usually thought of as an EN2 set), you could do 18 x 100 @ 1:45, holding 1:23 per 100. This takes about 30 minutes and allows :20 seconds rest between repeats - your job is to hold the pace for the entire set. Under most circumstances you will be able to do this because you know it is a valid pace. Another example set could be 6 x 400 @ 6:00, holding 5:39 per 400.
What is the margin of error for holding a pace? As evidenced in the above table, a 3% difference in pace will move you up to the next level of work. You should be as accurate as possible in maintaining your goal pace for a given set. This will take some experience - so don't be discouraged if at first you are "all over the map" when it comes to repeat times. You will learn what each speed feels like; how to relate your perceived exertion to actual speed. As you improve your level of fitness, and holding a goal pace becomes easier, it's time to repeat the test set to re-establish that goal pace.
Some days you won't be able to hold the "prescribed" pace. Why not? You might have been up too late the night before, skipped lunch, forgot to drink enough fluids, or still tired from yesterday's run. In these cases, you need to be good to your body and your brain - if you can't do the work, change it! Do some easy swimming, focusing on your technique. Come back the next time fresh and ready to do the work. Rest is part of an overall workout plan. Without it, you can't perform the work at the correct speeds to get the results you deserve.
Try this method to plan your workout pace for different sets. If you keep track of how you are doing, you can make automatic adjustments and updates to your paces as you improve. Almost any EN2 set that takes about 30 minutes where you hold the fastest possible even pace can be used to update your paces.
Swim On!
