Betagirl, congrats on signing up for the Aquathon! That sounds so very exciting! I was going to do one around here (Texas), but the entry fee is $175 (same price as doing the full Half Ironman), so I am passing on it.
You have so much courage and inner strength to sign up for something like this... and for cycling... when you have a disease that makes you feel weak and tired.
As Lise said, listen to your body... and do what you can do. It certainly sounds like you will do a fine job of finishing the aquathon.
Good luck and feel free to keep us posted on how the training is going.
"Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather, to skid in broadside thoroughly used-up, totally worn out, and loudly proclaiming: WOW WHAT A RIDE!!!!"