From StrategyWiki, the free strategy guide and walkthrough wiki

Only available after you have returned to Los Santos. The starting point is at the light house in Santa Maria beach and ONLY on Saturday and Sunday. The name of the contest is something that you'll find out if you look at a certain competitor. Be sure to have enough fat so that you don't burn Muscle. The order of the events are swimming, biking, and running. If you complete the burglary missions you'll get infinitive stamina making it a lot easier to do.

Contents 1 Swimming 2 Biking 3 Running 4 Reward(s)

[edit] Swimming

Once the race starts run and start rapidly pressing . You will swim around the light house and near a shore. Don't be discouraged if during the middle of this leg you get passed and fall to last, as you can make up a lot of ground near the shore. Once you are out of the water get on the mountain bike.

[edit] Biking

After you get on the bike, as before, rapidly press . The path will go from the paved two lane road to a dirt trail. Enjoy biking along and through the countryside and eventually through (literally!) the town of Angel Pine. Be in 1st or 2nd place by the time you cross the bridge near the near Missionary Hill. Once you get to the 1st intersection near the golf club, get off and start the next leg.

[edit] Running

After you get off the bike, run Forrest, run! You'll being running up the hill. If you begin to slow down due to fatigue then stop, look behind you, and if there aren't any others around take a 2-5 second rest. The Race ends when you get to the Radio signal next to the TV Building.

[edit] Reward(s)

For winning you get $10,000 dollars and if you needed to burn some excess fat it's most likely gone. If you get a low time, the reward can be higher ($20,000 is possible).