Swim distance,

Ideally for something like 10x100 with 15s rest you will have a set time for the repeat.  Maybe it takes you 1:45 to swim the 100, you get 15s rest and leave every 2:00.  Early in the set that 1:45 might not be very challenging but as the set progresses you have to work harder and harder to hit that 1:45.  If you can maintain it (or very close) for the whole set then it's a good target pace.  If you blow up and start doing 1:50-1:55 then you started too hard and next time try starting at 1:50 and go on the 2:05.  If you finish and that 1:45 isn't a challenge at the end, next time try 1:40 / 1:55

Little late to the party but I have to second Shiela T's 'Swim Speed Secrets, Workouts'. She has both a book and a set of workout cards, if you choose to only get one, get the workout cards. They have been a super HUGE improvement on my swimming. I now have structured swim workouts, not practice, but workouts.

You will not just get main sets, but you will get skill drills which will help build specific techniques and strengths. These workouts are based on 3 workouts per week for 8 weeks. The 3rd workout of each week is what is called a 'She Ra' workout and is pulled directly from her personal Olympic Triathlon training calendar. These are typically 4,000 yards+

For example my last workout Friday was as follows: This is Week 3 workout 2, Purpose: Endurance and strength through distance pulling and press outs.

Warm Up: 100 easy, 3x50 kick with board @ :15 rest, 100 drill choice (free/fly/back/etc), 3x50 kick with board @ :15 rest sprint the last 12.5 of each 50, 100 easy

Main Set: 3x 10x50 pull with buoy, Round 1 Descend 1-5 and 6-10 60-65%-90-95% effort, Round 2 Hold all 10 strong and consistent 80-85%, Round 3 25 easy 60-65% / 25 fast 90-95%  Plus 2x:30 press outs (up to 8 max) @ 1:00 (each of the 3 rounds)

Bonus Set: 8x50 kick 90-95% choose rest interval of :40-:55

Warm Down: 100 easy

Tubing Set 5 x 20 full pulls, Free or Fly your choice :100-1:30 rest between sets

Total Workout 2,200, 2,600 with bonus set + Tubing drills

So you can see that that the main set focuses strictly on arm strength and technique as the legs are simply floating behind with the pull buoy. (and the pressouts) Then after that the bonus set focus strictly on leg strength building.

That's one of the easy ones!!!! So if your swimming solo this is the ticket right here no doubt in my mind. My technique is getting so dialed in, I feel like a weight lifter after these workouts, my leg strength, my lats and triceps are making gains. Its crazy. And I can only get to the pool twice a week if I am lucky, typically I might only get in once a week. My pool is 35 miles across the desert in the opposite direction from my work so it makes it difficult, so that is why these workout cards make it even more important to me because each swim session must be maximized to its fullest value.

I also have my first half iron man coming in 3 months!!!! I can tell you I am behind the ball in training already.

Best wishes with your swimming progress, I can recommend these swim workouts with full confidence that just about anyone will see improvement in just a matter of a couple weeks time.

https://www.velopress.com/books/swim-speed-workouts-for-swimmers-and-triathletes/

https://www.velopress.com/category/swim-speed/