Thoughts on Pacing
Your First 1/2 Ironman
This pacing has worked very well in the past and is what I did to get to Kona, last year.
I was going to write a post for the 1/2 Vineman rookies, but this is a good opportunity to dump some thoughts for someone who is new to this distance or has blown up previously.
 Go out easy on the swim - the swim makes no difference to your overall performance. Use it as a warm-up for the bike. Two minutes faster on the swim can result in 20 minutes slower on the run. I ran past 250+ people at Wildflower last year. In an IM race, I typically pass 5-800 people with this strategy.
 Go out easy on the bike - your body will need about 6-10 minutes to make the adjustment from swimmer to rider. Take the first part of the ride easy in an easy gear. Initially drink water or highly diluted sports drink. Don't start eating until your HR has settled to your normal bike pace. It is okay for the HR to be a little high at the start but if this is the case then you should feel like you are pedalling VERY easy. Remember, it is a long day - there will be plenty of time to hammer later.
 About 15-20K into the bike it is time to start eating. By now you have let your HR settle and you have found a pace that feels comfortable. Personally, I will be racing Vineman at 10-15 bpm below my AT. On my first 1/2 IM I was 20-25 bpm below my bike AT. For your first race, remain aerobic at all costs.
 45-75K is, for me, the crux of the bike - this is where you should be fueling up and maintaining concentration. It is easy to get distracted in this period. Maintain concentration, maintain fluid intake and EAT.
 Overall, the purpose of the bike is to replace what you lost on the swim and prepare yourself for the run. There are ZERO benefits to hammering - let the hammerheads go. You will see them later [if you don't then they are faster than you anyhow ;-) ] Find a steady, comfortable pace. Stay aero, hydrate and focus. Remember that good body position is golden in a long race.
 Now the run. Start the run SLOW - are you noticing a pattern here? Many people do 1-5 above and then arrive at the run feeling great. They then blow their load in the first mile. Remember that you are about to run a half marathon. I normally leave a frozen bottle of drink at T2 so I can have a cool beverage to start the run. I run the first two miles real slow [please use your own definition here - right Craig?]. Normally, my stomach is full of food and water from the bike. Stitches are common as is a feeling that your legs will never come right. Believe in yourself, believe in your legs and they will come right somewhere between the 3-5K mark (assuming you listened to me about the bike!).
 Personally, I like to think about the run as really 4 x 5K. My strategy is to run the first 5K slowly. All I want to do is find my rhythm, hydrate and ensure that I am fueled up for the real race, about to begin shortly. Don't sweat your HR. The name of the game is getting your running muscles going.
 The second and third 5K pieces are where it all happens. You are still focusing on running steady. Here you can use your HRM to make sure that you don't run too fast and also make sure that you are not dogging it. If you are having trouble getting your HR up then get on the sports drink or cola if available. If your HR is running very high but you feel OK then this could be a sign of dehydration - water, water, water.
 Somewhere in the 8-16K region, you will have a period that feels absolutely awful (at least I always do). Stick with it. It will only last about 5-10 minutes and then you will be through it. Push through these problem times and you will get out the other side. Believe in yourself as an athlete.
 Hopefully, you are now around the 15K mark. You are tired but a bit stoked that things have gone so well. You can sense the finish line and you can do the math to see that you are going to beat your goals. You have run a smart race to here and will achieve/exceed your goals. Now it is HAMMER TIME. If you feel like it then rev your pace up. You will know the right amount to increase. Keep it aerobic but it is OK to get a good sweat going. Remember to continue to take fluids at every aid station, particularly around the 15/16/17/18K marks. Just ask PNF about the risks of skipping aid late in the game.
 Once you hit mile 12 (19K) spend everything you have, or simply enjoy the tailend of the race. I have done both.
As always, YMMV
For an experienced triathlete, I would agree with racing in the upper end of your aerobic zone. Normally, I feel like I am not moving at all and chugging along at my bike AT during the start of the ride. Then I settle and can start to work.
A few gordo data points for you. Back in 2000...
Max Bike HR ~180 Rest HR ~40 Reserve = 140 Bike AT ~165
I did most of the Tagaman bike at around 155-160 That is 82-86% of reserve for a 60K bike. I will probably target the same for 1/2 Vineman. That would be racing at an average of 5-10 bpm below my AT. I have confidence in my "engine" and in my ability to finish the distance. However, I know that this will make the start of the run just a little uncomfortable. :-o
I recommend a conservative strategy for most folks because it worked for me at Wildflower. Stats in 1999:
Max Bike HR ~165 Rest HR ~45 Reserve = 120 Bike AT ~150
I did most of the Wildflower bike in the 125-130 range. That was 67-71% of HR reserve. This made the run gravy and I ran past heaps of folks. A big mental boost. Remember that at this time, I did not have confidence in my engine or my ability to finish the distance. Going out conservatively paid big dividends and I beat my target time by 27 minutes.