What is your relationship with your bosses? It took me almost a year to get pregnant so when I did I wanted to tell everyone, but I didn't. However, I wanted my bosses to know in case my exhaustion and morning sickness, etc. started to affect my work (sometimes I'd be in the restroom for 20 minutes at a time). So, I told my bosses early on but asked them not to tell anyone else until I was ready. I also told the HR Director because I felt it was my responsibility as an employee to let them know. Obviously, that conversation was confidential and he too could not tell anyone until I told him it was okay.
If you don't have a good relationship with your bosses and/or you don't want to tell them your pregnant until your second trimester, I would just simply tell them you've decided not to do the ironman. If they ask you why, you could just say that you haven't been feeling very well lately. That way you have not lied to them and you have not misled them in any way.
Whatever you decide, make sure it is what you are comfortable with. Best of luck.