New and gasping for air!

Welcome!  

You should be fine for August.  I came into swimming like you, with running stamina - which doesn't translate.  Your lungs need trained and it's going to take work.  Try to get to the pool 3-4 times per week to really improve quick.

Here are a few plans:

http://www.beginnertriathlete.com/discussion/training/training-plans-view.asp?planid=13749&memberlevel=1 (10 week long plan)

http://www.beginnertriathlete.com/discussion/training/training-plans-view.asp?planid=13743&memberlevel=1 (20 week long plan)

--> You can click on the 'plan workout' button at the top to see the workouts.

These are triathlon sprint plans, but in your case, just use the swimming portion.  Once you finish with the swimming part of the plan, you can then build distance with an olympic plan.  This plan should be perfect for you.  I designed it with your backrground in mind.  

Let me know if you have any questions.

Welcome to BT Caroline!

You will find that many of us started the same way!  I have always swum, but when I first started training for tri, 25 m was all I could do without stopping to gasp for breath.  Keep working at it and it will come.  There are some great websites out there for swimming, Swim Smooth is one, Total Immersion is another.  Like Ron said, 3-4 swims a week, even if they are only 30 minutes, and you will see gains come.  Good luck!