It sounds like you are making progress. Progress can be slow. I used to swim breast stroke for 45 minutes to an hour. It did not raise my heart rate much. I cannot swim freestyle due to a tight shoulder.

I moved to the rower when the pool was getting repaired. 18 strokes a minute should be a good start if you try rowing. Rowers with electronic monitors will indicate your strokes per minute.

Keep it up and you should continue to improve.

The brain can take some time to relearn blood flow moderation.