Highs and Lows
It is inevitable you will experience highs and lows throughout your day. This is ok. It happens to everyone, including world champs.
When you experience a low moment in a race, have no fear. Just recognize it, know that it will pass and keep going. It is also a good idea to check in with your body when this happens. Meaning, when you start to feel like crap, ask yourself, "when's the last time I took in calories? Could it be time for a gel?" Often times the lows you experience throughout the day are a direct function of your brain acting up because it needs a little food. So, feed it!
Also, when you are experiencing a low moment it is normal to become annoyed easily. For example a slight head wind on the bike may really irritate you, but try not to let it. Remember, everyone else has to deal with the same headwind. Instead, change your mindset and turn it into a positive. "Wow, this headwind is very refreshing. Thank you Mr. Wind for cooling me down a little."
Finally, the highs you will experience are just as important to recognize as the lows. You want to be careful not to over exert yourself when you're feeling good.
For example, when you ride/run past a crazy crowd of wild fans (family, friends, teammate, total strangers), don't accelerate and show off. Stay cool, calm, collect and continue to race your race. Control yourself as this can bite you in the butt later in the day.
More: 3 Race-Day Tips From an Olympic Triathlete
Your Secret Weapon
This is one thing you should not leave home without. Having a secret weapon could possibly be the exact thing you'll need to get to your finish line. This applies to EVERYONE. Fast or slow. There will be a moment when your mind will tell you that it is impossible to take one step further.
This usually happens around mile 18 of the run. Sometimes before, sometimes after, but rarely never. So expect this point to come, and when it does you'll be ready to unleash your Secret Weapon.
You'll want to have something stored in the back of your head when this time comes. Why are you doing this? Are you doing it in memory of someone, for a charity or cause? Whatever it is, it has to be powerful enough to motivate you in your deepest, darkest moments.
This idea may sound silly but it is very effective.
As you know, your mind is very powerful. It can be your best friend or worst enemy. So take advantage of it. And use your head, not your fitness for the final portion of your race and don't forget to smile for your finish line photo!
More: 7 Pitfalls to Avoid When Setting Goals
Search for Ironman race or triathlon.