Are you stopping because of being out of breath/heart rate too high or muscle fatigue?

Usually if otherwise fit people are having trouble with endurance in the pool, I would suspect that form and technique are still the main limiter. As an example, I swam with my sister last weekend and she hasn't swam in over a year. She quickly blew away almost everyone in the pool (including triathletes and Master's swimmers), even though she is not conditioned right now. Her form however is quite good.

Are you working with a swim coach? That would be my first suggestion. How many strokes per length does it take you? If it is over 20 (for a 25 yard pool) then technique is definitely limiting your efficiency and thus your endurance. Length of glide, body position, and correct catch and pull are some of the things that have a big effect on how efficient your stroke is.

Where ia your muscle fatigue? In the shoulders? In your back? If you are lifting 2-3 x per week are you lifting for strength and power or doing light weights with higher reps. If you tax you lats before hitting the pool you will have a harder time. I assume your legs are sinking as well which is causing you to pull yourself throught he water with your upper body - seeking some form correction as mentioned in another post will assist in this area. Also try lifting after swim days not before and see if this helps.

good suggestions. in terms of lifting i don't lift on swim days but i will often lift the day before or after. i'm not power lifting and i only do one shoulder exercise as to not overwork the shoulders. I think that getting some lessons is a spectacular idea...

Jammers will make you somewhat faster, but psychologically they might make you feel a lot faster.

Some lessons and following a good beginner workout program will really help. I know this program has worked for a bunch of new swimmers.

http://www.personal.psu.edu/faculty/e/x/exk7/SwimWorkouts.html/ZeroTo1mile.html

yes, at least until you get a good form. wearing drag suits does have benefits but i wouldn't recommend it until you are comfortable with your stroke. it's like playing basketball in hiking boots.

get a speedo, like UFTrigator said, or jammers. i'm an old skool speedo guy, too bad my old ones don't fit anymore!

I agree with Ironmom IT IS MOST LIKELY TO BE POOR TECHNIQUE. The shorts or pants won't matter that much and I regularly do weights on the same day or day before swimming and it doesn't matter that much. Unlike biking and running where miles matters most. In swimming technique does. You can swim heaps and not really improve because you are just practicing the same mistakes over and over again. In swimming you need to be become a "student" of swimming to really improve from the basic level.

What's 16+ hours on a plane? :) We're making it a vacation as well visiting some family in Brisbane and further north after the race. Looking forward to some good weather and some R&R aussie style.

mmmm Chipotle.