I am 3 years out from RNY (lost 190 pounds) and have successfully completed two half iron man races. Nutrition is the biggest challenge, hydration has never been an issue for me. A lot of trial and error and important not to listen to what other athletes are doing. The most important thing is to be adequately prepared for the length of time the event will take (8 hrs for a half ironman) and practice with the nutrition you will use on race day. I'm hoping to be able to do a full iron man in 2 years time. Good luck and remember it can be done.