As the name suggests, this mission is only for Franklin and it is only available during the day. You will have to participate in a triathlon against Mary-Ann, meaning you you have to swim, bike and run in a race. It definitely helps to have max Stamina, as you will need to stop and recover here and there if you don't.
The first section is a straight swim across the bay, which shouldn't prove too difficult, especially if you have high/max Stamina. When you get onto land, grab one of the bikes and start driving the course given to you. Don't get too worried if Mary-Ann gets ahead of you while on the bike, which she will likely do.
Keep pedaling and if your Stamina is spent to the point you're losing health, lay off and let it replenish, as there's still one leg left to the race. At the finish line for the bike portion, jump off and start running. If she's ahead of you, the foot race is definitely the point where you will be able to catch up and even surpass Mary-Ann. When you cross the finish line, the mission will be over (whether you won or loss).