For my HIM, I wore tri shorts, a sports bra and a tri top for all 3 legs of the race. I must have a pretty tough butt because I've found that I like the tri shorts. I haven't done a full century in them yet but plenty of 60-70 mile rides.

Everyone is different and there's certainly time to do a quick change of clothes for that length of race. You just have to experiment during training to see what's right for you.