40 second laps is probably closer to a 12 minute sprint distance. 15 minutes is closer to 1:00 per lap. So you're probably just going too fast. Slow it down a bit and see how you do. I do a sprint in around 15 minutes, and can probably only hold a 40 second pace for 2 or 3 laps. So that sounds about right.

Edit: You're saying half mile: is that a longer swim than the normal 750m, or are you calling 750m a half mile? just to be clear, I'm talking about a 750m distance swim.

When you did the 1/2 mile swim in the race did you wear a wetsuit? Wetsuits can make a big difference in your time and make a slow swimmer much faster than they really are.

Otherwise I would just suggest slowing down in the pool and focus on your techinque to help distract you from the boringness (is that even a word??!?!) of the swim. I actually hate swimming in a pool and find it much more difficult than swimming in a lake or pond.

when you're starting to feel either fatigue or lack of air while swimming your laps: slow down. Move slower. Shift down a gear and take slower strokes and deeper breaths.

Don't allow yourself to go only one speed which may be too fast and forces you to stop. Adjust the rate at which you are moving. Be calm and listen to your body and communicate with it by making adjustments to your effort level, breathing volume and rate and stroke cadence throughout.

It took me a long time to learn this because I was a swim or stop guy and I learned to listen to my body and cooperate with it to keep going.

I suspect that your breathing technique is more to blame than some lack of physical endurance.
Here is what really helped me swim over long distances without having to stop:
Exhale continuously through only the nose when your head is submerged. Exhaling fast through the mouth can empty your lungs way too fast and cause you to need to inhale faster. Do not hold your breath in. Always be either inhaling or exhaling - one or the other, never holding it in.

Exhale deeply. Keeping too much "stale air" In your lungs by taking shallow breaths, whatever buoyancy it may give you, will increase your CO2 levels, limit your O2 intake, which will make more likely to gasp.

Let your stroke rate be driven by a comfortable and sustainable breathing rate rather than trying to set your breathing to accomodate whatever arbitrary stroke pace you set.