Oh, the swim...

Just keep at it.  Swim 4-5 times a week until the tri.  If you're swimming often enough, 400m will seem like nothing in a month.

Is it a pool swim or an open water swim?  If it's a pool swim, you can always stop at the wall and let people pass you, catch your breath, and keep going.  If it's open water (and you're wearing a wetsuit), you can always stop and float to catch your breath.  If no wetsuit, you can stop and tread water and get re-adjusted.

I don't see anything in your logs, what do you typical swim workouts look like?

think about long gliding stokes and slow down (best swiming advice I ever got) you will actually go as fast if not faster. If you yourself getting close to that point slow down a little

If you've only been doing the front crawl so far, try mixing in some side stroke or breath stroke. I can do breath stroke for miles, albeit at a very slow pace. i noticed big gains after a few sessions with the kickboard... try doing 25s with the board in between 50s of strokes. good luck with the first tri!

I agree with one of the previous posts, slow and steady wins the race.  Use long, slow gliding strokes when you swim.  When you pull and get ready to breath turn to your side and glide.  This will make you slimmer in the water and make it easier to get a good breath.  I recently started working on this and I took off nearly 3 minutes off my 500 yard time.

 You can make a *lot* of improvement in endurance in a month.  You can get to 400m.  Just put work into your swim ~4 days a week.   No need to freak out.

Sounds like I'm on the same track as you.  August 9.  First time.  400 yard swim.  Starting from a running base.  My bike just arrived in the mail.  I don't swim particularly well.  I've found a lot of inspiration and information on the website.  If you are enjoying training, keeping at it will get you there.

I think you mean 'breast stroke'   , but I agree -- I did in the past a few times switch to breast stroke to catch my breath.

I would invest in the Total Immersion  book "Triathlon Swimming Made Easy"

 http://www.totalimmersion.net/store/home-page/triathlon-swimming-made-easy-the-total-immersion-way-for-anyone-to-master-open-water-swimming.html

One probably needs a Master's Swim Class to get to the next level, but to get comfortable this TI system works.

I started in November with 100M getting me totally gassed.  Now, my longest pool swim to date was 2600 M in just under an hour.  I am not fast, but I am titally comfortable.  The book is $19.95 and definately worth it.