Help me figure out my DNF, please!

You were probably low on electrolytes (sodium and/or potassium). 

Was race day unusually hot or windy or were you working significantly harder in the race than what you've trained? 
Do you regularly ride mid-day or do you do most of your riding early morning and evening? 
What was your nutrition and hydration for the DNF? 

Sports drinks provide a significant amount of sodium.  If you prefer water, you can pick up salt tablets.  The challenge will be you haven't really practiced these on your training rides/runs and race day is a bad one to experiment on. 

I'm no expert, but my initial gut reactions says that you didn't have the right fitness level.  You don't have anything in your logs so I couldn't verify if this was true or not.  Main questions:

1. How often were your workouts?  What plan were you following?
   
2. How long were your longest rides?
3. Had you ridden over 40 miles before?
4. What's your background with triathlons?
   
5. How was your nutrition during the bike?  How many calories did you take in?
   
6. Were you drinking enough?  How warm out was it?

Don't take it the wrong way, I'm just trying to get an understanding of where you currently stand.

 I get the chalky white salt stains on my long workouts, its not uncommon.  Things like shot blocks and gels have electrolytes, but you can supplement them with salt licks and endurolytes from Hammer Nutrition (that's what I use).  People debate their effectiveness, but for me they work pretty well.

The salt is from your sweat.  As the water in the sweat evaporates, it leaves behind the salt.  If you sweat a lot and enough of the water evaporates, you are left with the "chalky and white" salt residue.  It is not a sign that you need to take more salt.

If you've trained well and you pace yourself well, I'm sure you can have a more successful outing at your upcoming HIM.  Good luck!

Right...mostly it's a sign that you're working hard and the relative humidity is low.

Do you remember what the weather was like during your half?

thank you for you responses...I've made notes and have a short but important laundry list.  I thinks it was a combination of several things brought up here. 

1) the race was late in the season (OCT) and fitness level may have tailed off.

2) It was Hot, dry and windy...I'm from Dallas so I'm used to that...however, I was racing in Austin and I'm guessing my hill training could've been better.

3) I posted great results and shorter distances last year averaging about 24-25mph at oly distance...i was trying to simulate that went out too hard/too soon.

4) My areo bottle was filled with WATER instead of sports drink so I'm guessing I was running low on elecrolytes.

I was porbably a little overconfident since my first HIM was on flat roads in super-favorable conditions...  I guess conditions are everything.  I felt great and breezed through my first HIM, but my second...wow what a difference!

I'll take it easier on the first half of the bike this weekend and then if I feel good, I'll push on the second half...

Any other suggestions?

Nothing wrong with the water as long as you are getting nutrition from somewhere (gels, bars, etc.). 

Pacing was most likely your mistake last year (possibly aggravated by weather conditions too, though it sounds like you would have been acclimated to those).  It's almost always better to go "too easy" on the bike and make up for it on the run.  The reverse just doesn't work.  The more you push the bike, the more you risk going over the edge of your current fitness abilities.  Once you reach that point in a long course race, well...you now know what can happen. 

Hopefully you've had a chance to practice your pacing in some of your workouts and have a good idea of what kind of effort level you can maintain through the swim/bike and still run reasonably well.

Sounds like you may have answered your own question, but the main reaisn people have trouble in races that didn't happen in training is they go too hard in the race relative to their fitness and training.

You mentioned that the course was hilly but tried to go as hard as you did in a flat race? I know that powering up hills and then running can make your quads feel all squirmy and climbing will place challenges on your lower back which might explain why being aero hurt.

Without some sort of blood test we can only speculate about your sodium levels. Sweating out salt in itself doesn't tell us anything, or what your salt levels were like going in. There is some debate right now about how much  electrolyte a person really needs in a race, and there is compelling evidence that athletes don't need as much as we think they do, although there is much anecdotal evidence on either side of the argument.

My first guess would be you went too hard on the climbs and you were fried by the run. You're going to have to think back if you were drinking enough and perhaps simply dehydrated, too, which will certainly make you feel crappy.

To answer your question about getting salt: extra salt on your food leading up to the race. Sports drink has plenty of sodium. If you want sodium without the sugar, you can get dissolving tabs such as NUUN you can also pop salt tabs. Some racers have complicated hydration methods and some seem overly simplistic.  You're going to have to figure out what works for you.