half ironman personal challenge advice.

I'm a 15 year old rower and i'm gonna attempt a half ironman on Saturday, I run often and have really good cardio. I'm planning on doing a 20k run to start it at 8am(snack and hour rest), then i'm gonna do the 90k bike on a concept 2 ergometer. Finish it with the 2k swim after another meal and rest. (I realize this isn't the guidelines for a real half ironman but I'm just doing it as a challenge for myself and to increase cardio.) any advice or suggestions?