Wow Hulagirl. Plan on doing rtr after an event like the ironman. That could be brutal. Or not. I have done three rtr's and live in CO. Altitude is certainly an issue. Weather wise, it is variable. Most years it is brutally hot by late June. (Not this year.) It also depends on the pass locations for weather. Leadville is always cold in late June. Trailridge also very cold. anything over 12,000 feet probably still cool. I don't overinsulate, but protect from wind. Light wool layers and rain jacket, goretex gloves (no insulations) maybe arm warmers. Maybe a skull cap.
Enjoy the planning and training.