Improving Swim Endurance

As a beginner it is a reasonable certainty that you have little swim fitness, however you likely have a bigger limiter than your fitness.  Swimming is highly technical.  At this point in your swimming career, you are probably "beating the water into submission," rather than swimming.  If you have poor technique you will have a very difficult time building fitness and becoming a faster, more efficient swimmer.  

The very best thing you can do is to get some lessons.  Spend some time with a qualified coach/instructor and learn the fundamentals, then progress to swimming freestyle.  It's valuable to learn all four of the competition strokes but since freestyle is the fastest stroke, that's the one you will be focusing on.  You can possibly find a coach or instructor at the local YMCA, perhaps a local swim team.  Absent those options, there are countless videos on-line that can help.  the problem with videos is you won't know if you are "doing it right."  Having "eyes-on-deck" in the form of a coach or instructor is the surest, fastest path to swimming with proper technique.

Swimming technique and swim fitness are two sides of the same coin.  You can't have one without the other and thus, must be developed simultaneously.  As you begin to learn and swim with proper technique, you will begin to build swim fitness based upon that proper technique.

When you begin swimming actual workouts, 16 x 25 w/20-seconds rest is better than 1 x 400.  During the intervals you will be resting so you can maintain proper technique.  During the 400, your technique will break down due to fatigue.  You want to store proper technique to muscle memory - doing intervals allows you to do that.  As you become stronger, you can add repetitions or change the length, i.e. you could go to 10 x 50 or 20 x 25.  As you continue to get stronger (and faster) you would continue to lengthen the workouts.  You DON'T need to do long continuous swims in training.  If you feel the need to do an occasional long time trial so you are mentally confident you can swim the distance, that's understandable.  However, long swims in training create problems - your technique WILL break down during a long swim.  Remember - you want to store proper technique to muscle memory - doing long swims does the opposite.

Hope that helps.

I started swimming a little less than 4 months before my first Triathlons witch was a 70.3 that has the starting line 10 minutes from my front door. I couldn't even go 15 yards with out panicking for air and having to stop regroup and start again.  For me it was NOT endurance that was the problem but rather form.  I search for breathing drills and the next two weeks I didn't even attempt to swim a full length of the pool when I was doing my swim training.  I spent all of my time on the wall doing breathing drills.  After I got the breathing figured out things progressed quickly.  I initially was not able to swim really far, but I made goals in increase every time I was in the water.  Swimming is a sport that is all about form. I wouldn't worry too much about increasing distance I would focus a lot on form drills.  Can you kick on you side for 2 laps? If so that would be a good place to start.