Triathlon - Tri Noob - Swim Questions

I'm starting out by doing a "Try it on" Triathlon (300m swim, 9 mile bike, 2 mile run). Not very intensive, but I wanted to get my feet wet. I'm in decent fitness shape and am running 3-4 miles at 10:00/mile pace and building. I'm new to cycling also, but I think i can build for a 9 mile ride, as i have some friends that can help me and ride with.

My question is to swim training....i'm a decent swimmer (never competitive), but i'm a little new to swim training. I've done about 3 sessions so far and i've tried paying attention to my form, over endurance. On Sunday, i went for an open water swim at the lake and was very winded about 100m out doing freestyle. I had no problem doing the 300m, but of course i had to do the breast stroke and even backstroke for a minute or so.

What's the best way (training) to build endurance in swimming to make the swim without getting winded? Of course i want to have good form also, but would like to be able to swim the full 300m freestyle and build on that afterwards.

When I return to swimming with low fitness, I always start with short repeats because when fitness is low, my form falls apart very quickly. So my first session might be swim 25 yards, rest 10 or 15 seconds, and repeat. Do that for 20-30 minutes. As fitness improves, I increase the length to 50 yards, then to 75 yards, then to 100 yards, etc. and I also gradually increase the workout up to an hour or so. After a few weeks, I usually have fitness to do a "real" swimming workout (like you might see in a swimming magazine or book or website).

I assume you meant breast stroke rather than butterfly.

Gonzo Bob is right on with his advice. To that I'll add do your training in a pool if you can at first. You can pick up on improving your form watching and talking with others as well that way.

Here are some links to some workouts you might try.
The biggest thing for me was interval training.... Work yourself up to be swimming up to 400-500m in a workout without stopping.
Also, remember that the 300m is going to be from a buoy to buoy and wont account for the swim out to the buoy and in to the shore. You'll run into the water...swim out to the buoy (probably 20-25 yards) then swim the 300 course...then turn in and swim back to shore until you can stand then wade back in.. (another 20-25yrds)

Good Luck
http://www.beginnertriathlete.com/KevinTSC/Interval_training.htm

http://www.beginnertriathlete.com/Priscilla/Triathlon_Swim_Training_For_Beginners_Dec03.htm

http://www.triswimcoach.com/