Instructions
Build your stamina, cycling and swimming skills. These skills go up whenever you run, ride a bike or swim in the ocean. If the skills are very low, you are not able to start the race.
Wait until Saturday or Sunday in the game. Press "L1" to find out what day it is. The races are available at any time during these two days.
Go to Los Santos and head to Santa Maria Beach. Run toward the lighthouse and look for a glowing red marker. Stand in it to start the first race.
Play through the game until you complete the "Yay Ka-Boom-Boom" mission in Las Venturas. This unlocks the second race.
Go to Fisher's Lagoon in the Red County area. Look for the red marker by Palomino Creek when the weekend arrives. Walk into the marker to begin the last race.