Help with endurance swimming

Will,

Congrats on learning to swim. Getting out of your comfort zone says a lot. I'd recommend structuring your pool time. I'd focus on breaking your time into 3 phases. Warm up, main set, cool down. Seek a local masters team if you want to get better. A typical workout looks like:

Warm up
400 W/U mainly Free, but you can work in other strokes
250 drills mixed (depending on what event and time of year) These include 3 stroke 6 kick, 1 arm swimming, lots of dolphin kicking on front and back, swimming on back for 12 kicks then turning over on front for 12 kicks hands at your side, free breathing every 3, 5, 7, 9 strokes, catchup drills.
200-500 pull work with buoy with fins and paddles focusing on reach and hand entry.
main set
4 or 5 x something like 100's, 200's on your interval time
Wednesdays are always sprint days so we do 3 x 10- 25's or 3 x 5 - 50's.
Sometimes just a lot of kick drills or IM work
Cool down
200-500 pull or slow free breathing every 3,5,7,9.
Most workouts we do anywhere from 2k- 3k in an hour.

The key to pool sessions is swimming with a purpose. Always try to improve something whether it's body position, kick, catch and pull, balance. We never jump in and say swim for an hr. Our workouts are always targeting technique over volume. You'll build endurance faster if you do 10-25's on :40 seconds. This means leaving each wall on :40 seconds (not reaching the wall resting for :15-:30) then going again. Whatever your current interval is work with it. You should be able to get to 7 out of 10 before feeling out of air or gassed. If you're dying on say 4 then you over estimating your fitness. As you get better you can do say 5-100's or 4 x 200's trying to keep or improve your time over each set.

Again, I'd suggest looking for a local masters team and joining. Most will let you swim free for 2-3 weeks to try it out. It'll hurt your feelings both emotionally and physically since you've never done anything similar but stick with it. I've been there. You'll learn tons at a very low cost. You'll improve quicker since a coach will give you drills specifically targeted to your ability. Until you learn the correct technique which means doing drills (the correct way) and then using those drills which creates muscle memory.

Stay after it! Since you only learned 3 weeks ago don't get discouraged and set unreasonable goals. Do record your 25, 50, 75, 100 time and write it down for a baseline for improvement.