During your training build up to any long (2+hr) event, you should have a very specific idea of what you plan to do on race day.
Complete pacing plan - heart rate, power, perceived exertion, pace
While you can get away with just going hard and hanging on in 5ks, a sprint triathlon, or a short-ish swim or bike race, this doesn't work for longer events. At the bare minimum, you should have an acceptable range of effort level from the get-go. For example, in a half-Ironman or 70.3, an intermediate level athlete should target a heart rate range of mid zone 2 to mid zone 3, or 75-80% of FTP on the bike. For an Ironman, the same level athlete should target a low-mid zone 2 HR or 65-70% of FTP on the bike, and mid-high zone 2 HR on the run. The middle of zone 2 is very steady/moderate. More about FTP training here.
Course knowledge and review
I don't like showing up and having zero knowledge of the race course. At a minimum, review the course maps and elevation charts, and drive the bike course.
Strategic racing approach, if applicable
Advanced athletes may employ some additional tactics involving drafting on the swim, accelerations to drop packs on the bike, or surging at key break points in the run. Effort outputs like this must be rehearsed in training. Surging at mile 15 of a marathon is great if you have the capacity to recover. It is a real piano on your back at mile 23 if not.
Nutrition breakdown by type, time, range of calories - a consult with a nutritionist is good here
Winging it on the nutrition side in long events is not good, for lack of a better description. Way too many things can go wrong here beyond a bonk (glycogen depletion). Not only do you need a calorie per hour target, you need to make sure your body can handle that type of product(s) for 5+ hours. Some people have difficulty absorbing calories in during a run, so they need to front load the bike as well as possible.
Some amount of dehydration is unavoidable in most events. Minimizing that amount is critical, as performance drops off rapidly as dehydration increases. If it is warm/hot, you need to be ready to ingest more fluid than your cool training days. Making sure you can tolerate the product you are using is key, so some longer training days are necessary just to ensure this part of your plan will work.
You already know that water alone will not suffice in longer races. You also lose electrolytes. Athletes vary in this greatly. If you are a heavy/salty sweater, you'll have to do some serious planning, and then make sure you don't get off track on race day. I recommend a sweat test from a certified nutritionist/lab.
Caffeine plan, if applicable
Caffeine provides a bit of mental stimulation and awareness, and a bit of a performance enhancement. Consider using gels or products with caffeine in them. Rehearse your timing and total intake on long training days.
Mental strategies for adversity
The human mind and body can tolerate a great amount of discomfort. Get comfortable being uncomfortable in training. Use visualization techniques to review both positive outcomes and potential adverse situations like a flat tire, GI issues, competitor psyche-outs, and other things that can crop up. Teach yourself to stay positive and motivated by staying positive and motivated on your challenging training days. :) More here.
Total gear breakdown
If you got this far, you know there's a million things you need handy. Make a check list and have this all done 1-2 days before race morning. Don't let a forgotten race belt or pair of sunglasses stress you out on race morning.
Marty Gaal, CSCS, is a USA Triathlon coach who lives in the Triangle area of North Carolina. Marty has been coaching endurance athletes since 2002. You can read more about OSB coaching services at www.osbmultisport.com.
One Step Beyond is the producer of the Powerstroke®: Speed through force and form freestyle technique DVD, intended to help new to intermediate triathlon swimmers become faster and more powerful in the water.