Reply To: One set per session, slower swimmer
For the sake of time you may have to go with 50’s or 75’s. If you do 100’s and go for 30 reps on an interval around 2:00, that is 60 minutes right there not including the warm up. You would never have enough time to allow yourself to get to 30. You should start out with 50’s and work your way up to 75’s. Start with something conservative. Try to find a pace that is challenging by not super hard right off the bat. Go for 30×50’s holding 48 or 49 on 1:10. Once you make 20 before the first fail, move the pace down one second.
You could alternate 50’s and 75’s every other day to mix it up. Don’t worry about dividing your best time to figure out the interval. Start with something not too difficult and continue to move the pace down as you get in better shape and make 20 reps before the first fail.
Having only 1 hour may limit you to 500 training but that is okay. If you can condition you self to swim a good 500 with good splits and finishing strong at the end, you should be able to put in a decent 1650. Katie Ledecky has said that she doesn’t even really train for the 1500. She focuses on 400 training and can swim down to the 200 and swim up to the 1500.