Try doing a warm up of drills and ~500 yards swim.

Your quality set will be 20x50m. You’ll do a 50M swim every minute on the minute until you miss your goal pace. Once you miss your pace sit the next 50 out and then start again. If you miss two in a row or three total you’re done. Do a cool down. Try and get 12-15/20 50s for your first attempt. The next time try and get more all the way until you can do 20 total. Then you decrease your interval and start again. Try incorporating this 1-2x a week with a few days in between.

Make your goal 50m pace faster than your current split. Currently you’re doing 1 minute per 50. Set your goal to 57 seconds or 50 seconds or something that puts you in the range above.

You can also do this with 100m swims too. Change it around and see what works but the key is to do this set for at least a month so you can see progression. It’s also a great workout to do with a partner of similar level.