I have my first tri on the 27th with an ocean swim.  i'm not a swimmer by any means, but have been in the pool 3-4 days a week since i signed up a month and a half ago (i know, not a long time).  I've seen the total immersion videos and have improved my technique quite a bit, but i still have no endurance.  i struggle to do back to back lengths in a 25m pool.  i'm afraid once i get into the open water i'll lose all technique and start freaking out.  i did a mini sprint 3 weeks ago (only a 200m pool swim) and when i got out of the pool i was exhausted.

any advice, other than to just try to remain calm and go easy?

thanks for the advice.  the waves have really been picking up here lately, so i'm not getting much opportunity to practice there.  i know that will help me if the race waters are choppy, but i'm really hesitant about going out too far to get past the break by myself, and i haven't found a good spot in the intracoastal yet.  i've been using a pull bouy lately to simulate the extra bouancy of the salt water, plus it helps me not rely on my legs so much, in order to conserve energy for the bike/run.

i just need to focus that my only goal for this 1st tri is to finish, and times don't matter, and relax relax relax! 

Very common -- happened to me on my first (and second!) open water tris.

In addition to practicing in open water, other advise given to me was to count to 10 after the gun goes off before starting. Walk into the water and let everyone else sprint in as they scramble for a patch of water.   Let the crush go.  I tried this on my third and got the identical time as my second, but wasn't exhausted, didn't swallow any water and swam freestyle the whole way.  Alternatively, start wide to avoid the crowds.  Finally, don't worry about using other strokes if you need to -- I still use breaststroke intermittently to catch extra O2 and sight the buoys.

The best thing I have found is to "practice like you play". Even if you can't get out to do open water swims when you swim in the pool be sure to take slow even strokes concentrating on having your thumb come up and touch your thigh before you bring it out of the water. You don't want much leg kick and overall you just want to relax. The best thing that i have found for staying on course is to go to the opposite side  that you breath on. For example if you breath on your right side than go to the left of the course. This will let you look at other swimmers and you won't need to look up for the buoys so often. Warmups are a great thing because they allow you to get used to the murky water etc. Another thing that you can do in the pool is to swim with your eyes closed. First find out how many strokes it takes you to get accross the pool, then on your next lap close your eyes and swim 2-3 stroke short of that and for those last 2-3 strokes don't put your face back in the water, instead pick your head up and look at the end of the pool. this simulates you not being able to see and looking for the buoys. It also helps get you psychologically used to not being able to see while you swim. Another thing to practice is to breath from your non natural side, because sometimes you are getting hit in the face with waves from one side and being able to breathe from the other is a lifesaver. The biggest thing is to relax.

As far as being able to extend your swim workouts you have to just keep doing one stroke after another.