Creating stamina or endurance

Why are you swimming/what are your goals?  I'm inferring that you may be a triathlete, and I'm inferring that you hop in the pool a few times a week, and swim an unbroken mile.  Swimming is unlike running and cycling in that stamina and endurance isn't just cardio, there is definitely a muscle fatigue factor that greatly exceeds that of either running or (road) cycling.  Swimming got away from hours and hours of low effort, "Zone 2" yardage similar to how cyclists train 80% of the time.....there is even a term for it, "Garbage yardage."

I'd suggest you start going interval workouts at a higher intensity, making sure you have sufficient rest between reps.  There are two things that I would suggest will help you know how you are going, one is your split times, the other is stroke count.  And I'd say stroke count is more important.  When you start tiring, muscles here, not cardio, your stroke count will suffer.  If you do a mile, you may start out taking 15 strokes per lap, and may end up taking 22.

Here's an example of a set I'd recommend.  It is far longer than a mile, but I think it is what you need to do.

Warm up - 400 yards swim, 200 yards kick - NO FINS, 400 yards pull (paddles and pull buoy).  Easy pace here, you are warming up.  Pull sets gets those shoulders and lats engaged.

Main set - 40 50's on :20 rest interval.  COUNT YOUR STROKES.  Example, you are hitting :40 splits taking 17 strokes per lap*. Your goal is to hold that as long as possible.  You'll probably do the first 5-6 in something like :38 with 16 strokes, but then you'll settle in.  So you'll swim a 50 every minute (maybe 1:05 if you come in at :45, maybe :55 if you come in at :35, maybe :50 if you are coming it at :30) settle in, pay attention to both stroke count and split time.  If you have to increase your stroke rate to keep the same split time, you are probably losing your technique.

That's not an ideal set to swim multiple times a week.  It'll get boring.  There are several sets posted here:  https://www.usms.org/workout-library  You'll find a triathlon section, too.  Many of the workouts will have shorter rest intervals than 20 seconds, and as you build up stamina, you can likely switch over to those, and they are likely to suit you better.  But until you can build some muscle stamina, and be consistent with your stroke count, you will be better served to develop your stroke with more rest, so it doesn't fall apart.  Insufficient rest leads to poor technique, which leads to bad habits.

If you are finding that you are adding more than 1-2 strokes through the set, then increase your rest interval a little.  But you should get in a groove and hold it.  Focus on grabbing a lot of water, and not tempo.  But the rest between reps will help you recover, so that each rep works your strength, not just your cardio.  Without the rest, your muscles will fatigue and you'll tire, and your stroke will fall apart.  Your stroke count is the best indicator of your fatigue.  Sort of how when you get tired on a bike, you tend to want to spin more uphill than mash.  In swimming, you are a masher, not a spinner.  Spinning as a swimmer is horribly inefficient.

*When swimmers use the word "lap," they mean one "length," NOT two.  25 yards in a 25 yard pool, 50M in a 50M pool.

First I want to thank you for your detailed explanation. You have provided a lot of information that I can learn from.  But, I apologize for not fully explaining my level of swimming and what my abilities and goals are. I am not a triathlete. I am a 72 years old who started swimming 3 years ago and have been teaching myself since the beginning.  I do swim a mile 4 times a week but it is not nonstop. I warm up using a pool buoy doing 200 yards of freestyle and 200 yards of backstroke. After that my level of stamina allows me to swim only 50 yards without the pool buoy. My goal is to continue swimming and to improve my stamina and endurance. I just completed a US Master Swimmers training for technique. I focus on freestyle and backstroke at this time.

Wow, that's awesome!  SOrry I misunderstood where you are coming from.   Taking it up at 72 is inspiring, I think it is great!

I think if you can find a Masters team to train with, that may be hepful.  Try to talk to the coach, and maybe see if they could work with you privately on some technique.  But overall, what I would say is that you should ditch the buoy, and rather than resort to using it when your stroke starts falling apart, increase the rest interval between 50's.

One big thing that a lot of beginners get way wrong, and I did this when I took it up 7-ish years ago, is breathing.  Don't hold your breath.  Breath in, and as soon as you put your face back in the water, start exhaling slowly through your nose.  Holding your breath, and then trying to exhale while you have your head turned, will absolutely destroy your ability to breathe, and thereby stamina.

Wow, that's awesome!  SOrry I misunderstood where you are coming from.   Taking it up at 72 is inspiring, I think it is great!

I think if you can find a Masters team to train with, that may be hepful.  Try to talk to the coach, and maybe see if they could work with you privately on some technique.  But overall, what I would say is that you should ditch the buoy, and rather than resort to using it when your stroke starts falling apart, increase the rest interval between 50's.

One big thing that a lot of beginners get way wrong, and I did this when I took it up 7-ish years ago, is breathing.  Don't hold your breath.  Breath in, and as soon as you put your face back in the water, start exhaling slowly through your nose.  Holding your breath, and then trying to exhale while you have your head turned, will absolutely destroy your ability to breathe, and thereby stamina.