While the swim in a half-Ironman is insignificantly longer than the swim in an Olympic Distance race – 1.2 miles versus 1 mile – your strategy should be quite different. After all, you will be racing approximately twice as long. The sense of urgency you feel in an Olympic Distance race should be toned down or absent during a half-Ironman race.
When the gun goes off, you can sprint the first couple of hundred meters until the pecking order is established, but your goal should be to find your place in the drafting line as quickly as possible. This will help you minimize the time you spend in an anaerobic state and translate into more energy and speed saved for later in the race. Once you settle in, your HR should be somewhere in the 65-78% range. If you are comfortably drafting behind someone and feel like you're going along at warm up speed, that's fine. You can either stay put and channel that extra energy into the bike, or you can try to break out and move up a few places in line (or bridge the gap to the next pack of swimmers). But use caution here. It is all too easy to expend too much energy during the swim. A couple more hours into the race, this can come back to bite you in the rear end. You should exit the water feeling slightly winded but like you could also hold that effort for another 1.2 miles. If you're as tired as you are after an Olympic Distance swim leg, you swam too hard.
As you transition to your bike, take the time to keep your HR under control. Losing a few seconds here will not alter the outcome of your overall race. Poise and patience pay off. Be aggressive, but do so within the context of the race ahead that still remains.
The Bike Leg
The bike leg of a half-Ironman lasts longer than the time it takes to complete an entire Olympic Distance race. If you are able to hold a steady pace, then you should pick a level of effort that allows your HR to start out in the 65-78% range and gradually drift up into the 76-86% range. From a power perspective, you're looking at building through a range of 70-85% of FTP. You will read some suggestions that you can go up to and even above 90% FTP. I caution against this because it is very easy to overshoot your effort early on when you're fresh and feeling strong. Better to start in control and conservatively, then be able to speed up and either even split or negative split the bike leg. Start out too strong, and you will be slowing down, thus sending your race in the wrong direction results-wise.
How long you spend above 78% HR or the upper region of the suggested power zone completely depends on how long your race is going to last. Remember, once you tip the balance toward an anaerobic effort – something above 78% - you start to quickly limit the time you can effectively compete at that level of effort. With the run looming ahead, you don't want to expend too much energy on the bike. When you think you have about 2 hours left to race, you can fully be above 78% HR and not worry about negatively impacting your results given you are fueling and rehydrating properly.
Keep your cadence at 90-100rpm. One pitfall athletes fall into is spinning too slowly given the lower level of effort. This only serves to deaden the legs and rob them of energy you'll need on the run. Similarly, if the course is hilly, you should try to spin at 85+rpm on any hills and make sure you keep your HR under control. Sharp spikes in HR can cause massive amounts of lactate to flood the muscles and derail any chance you might have of a solid performance. Likewise, be sure to pedal on the down hills. This helps keep things flowing through the muscles and keeps them fresher than if you simply coasted downhill.
As you near the transition area, shift into an easier gear and spin a little higher of an RPM for the last few miles. This will further help your legs prepare for the run. You won't lose much time (if any) and you'll be better primed for a solid run. Your HR should drop a little bit, which will serve as a brief rest before you dismount. As in the first transition, take your time here. You're about to run a half-marathon, so take the time you need – whether it be seconds or minutes – to properly collect yourself, take a mental inventory of how you're feeling, to squeeze a gel into your mouth or take some sips of a drink, etc. Get your running shoes on and head on out!
The Run Leg
Your legs will probably feel somewhat heavy, since by now you've been racing for the better part of 3 hours or more. Be patient and let your muscles stretch out over the first mile. Let your stride lengthen naturally and, before you know it, you'll fall into a comfortable rhythm. At this point, you'll probably have 90+ minutes left of racing. Given this, you should start the run in the 70-78% range until such time as you've taken inventory of how you're feeling. Once you've got roughly 90 minutes of running left, then head right into the 76-86% range. Don't jack your HR up to the top of the zone, a recipe for disaster. Build up your effort and let your HR gradually rise. Fall into a rhythm and pace that situates you in the middle of the zone – in the high 70s to low 80s of Max HR. On any up hills, gear down your effort to keep your HR under control. As with the bike, you want to avoid sudden, major spikes in your HR to avoid excessive lactate production.
When you feel you've got about 30 minutes left to race, if you're feeling strong, you can turn it up a notch and hit the 84-92% range. If you were running fresh, you could hold this level of effort for about an hour. But since you've been racing for at least 3.5 hours at this point, you're already tired and you're already pretty depleted. Best not to be too aggressive too early. Again, gradually build up your speed to enter this higher HR range. Hold steady and kick it in to the finish.
Your level of effort during a half-Ironman should be thought of as stair steps. You are doling out your effort and gradually raising your HR so you don't overdo it too early in the race. You can always speed up if you have extra energy left. However, if you burn your engine too early, the only thing you'll be doing is slowing down.