Congratulations on your first tri! Feels pretty good, huh?

I know that you are feeling tired, but I agree with the others that posted before me today. Were you training with a group? If so, call one of them and arrange to meet for a short workout. You will feel better after you get out for the first time, even if it is for a short run. And in this heat, a jump in the pool for an easy swim after your run will feel wonderful!

Good luck.