Whats the best way to build up swimming endurance??

Yes, there is plenty of time for you to swim a mile by Oct. You have the right idea with 3-5 times per week. More time in the pool will do wonders for you swimming.  Without logging your workouts I don't know how much time each pool workout is but here is some general advice. (Take it with a grain of salt if you like.)

Try to start each workout with a 10-15 minute warm-up doing any stroke. Then work on some drills, doing 25 yds of drill and 25 yds swimming. Perhaps 5 times to start. For a main set do 50's or 100's trying to keep your stroke long and taking 20-30 sec rest after each one. Then a cool down. If you have the time, each session should be 45 min to an hour. You don't have to do long distance swimming to increase your endurance at this point. It's more important to improve your stroke efficiency.

There are some video links on my blog if you want to see some visual aids. Good luck!!

Your logs show no activity. It's kind of hard to give advice when those offering advice can't see your past performance. I understand your reluctance to post them for public consumption (it took me a while as well), I'm just saying...

Anyway, without seeing your logs and assuming you just want to finish the race I'd say over the next week work up to 2000 or so yards/meters, broken. Which is to say 20 100's or 10 200's with minimal rest (10-20 seconds) in between. Once you can do that try 4 500's broken then 3 700's then 2 1000's then a 1500 and a 500. Then a 2000 stright. It wouldn't hurt to get a few 100's warm up and cool down in as well.

If you write up a plan for the next 7 weeks with your breakthrough workouts being these longer distances I think you'll be able to make the swim. Good luck.

Thanks for the tips guys.  I actually do log my workouts, Ive just been lazy and havent done it in 2 months, my fault I know. What I'm trying to accomplish is to complete an Oly without having to be drug across the finish line or be taken to an ER after the race.   I appreciate the tips though, extremely helpful.

P.S. Thanks for the hand-slap, I'll start logging my workouts again,

ZERO to 1650 in Six Weeks

Worked for me and a lot of others here on BT.

You swam 800 meters in 21 minutes, so it appears you're probably technique-deficient. Until you can do steady-state swims at <2:00/100, your best bet is to focus most of your swimming od nrills and technique. Once you get more efficient in the water, you'll find that you can swim longer distances with much less effort. And that will greatly decrease your times.

I tend to disagree with the "practicing bad form" theory. The more time you spend in the water the better you will swim. Count your strokes and try different drills that will get your stroke count down, it's that simple.

Don't get too technical with your stroke. The key to swimming efficiently is to swim relaxed. Start off by counting to 2 before starting your pull, that alone will lengthen your stroke.

One thing I have learned by teaching adults to swim is that they want exact angles and catch points, degrees of rotation, etc. Just relax and swim and try to pay attention to what gets your stroke count down.

You dropped your time from 21 to 15 minutes, showing in my estimation a serious improvement already.  My guess is that most of this was technique improvement, not fitness improvement.  So have some confidence that if you can swim half a mile in 15 minutes, a mile should only take you about 9 minutes longer than your old half mile time... and what's 9 extra minutes when you've got 7 weeks to build up to it.

I agree you will continue to see your greatest gains by working on form.  But I disagree that you'll need to see a coach 3 times a week to get there.  Often, just one session with a skilled diagnostician/coach who can point out some breakdowns in your form and recommend some drills to improve those will make dramatic improvements.  I've spoken to more than one person who just by watching the olympic swimming on TV this past week noticed that something was off with their form (not catching early enough, etc) and instantly saw a swim improvement in the pool by being conscious of this deficiency... and taht's before drills to work on it.

So in summary - you'll be fine and I think definitely be able to accomplish your stated goal by October.

Opps double post.

Take a look at the BT OLY training plans.  I am doing the 16 week OLY plan.  Looking at the swim workouts, they peak with a 77 minute long swim workout.

You should increase your time in the water with each swim workout. I do one long swim workout where I pace myself and increase the time each week.  Then one short workout where I do drills and/or intervals.

Make sure you do some drills.  My favorite is the closed fist drill.