Transplanted from Atlanta and now successfully living in Boulder for about three months, I’ve collected several observations and suggestions for those participating in Ironmans Boulder and Boulder 70.3, as well as for those simply visiting this great state! Note that all of the following is un-researched and unsubstantiated. Simply Susings…

Hydration

Whereas you might find yourself drenched head-to-toe merely stepping outside in the Southeast, in Colorado, you may not even notice your sweat during an activity as intense as Ironman. Nope, the arid climate sucks the sweat from your skin, so you’re likely losing far more fluid than you expect. Likewise, your “imperceptible” fluid losses from breathing will be higher. So you’ll need more fluids to stay hydrated than in climates with more humidity. And don’t forget about those electrolytes to avoid stuff like exercise-associated hyponatremia.

Wind

Depending on the day, it could be super windy with some pretty intense cross winds, so this might influence your wheel selection. I’m a huge proponent of discs, but even I would be nervous to use one if it happens to be a windy day.
Also, there inevitably seems to be a wind combined with a false flat terrain, which can be demoralizing. Remember that everyone else is experiencing the same! So tuck yourself down in your aerobars and hang in there. Because it, too, shall pass. Well, actually, the winds seem to always change to orient themselves in your face! But just enjoy the ride regardless.
Pace yourself!
While I’m told that the altitude slows your run pace by 7 seconds/min and reduction in wind resistance from the thinned air negates any negative effect on cycling, my personal experience is that my Z2 paces were a full minute per mile slower running and a full mph slower biking upon arrival at altitude. So take that into account when you get here and adjust your goal paces accordingly so that you don’t blow up! You might want to pay close attention to how your HR responds during shake out workouts. And arrive as early as your life allows to acclimate as much as possible–a process that takes three months to fully transpire.
Look up!
Wherever you are in Colorado, it’s BEAUTIFUL!! So enjoy the opportunity to soak it all in while you slow down the pace! Perhaps the lack of oxygen is Mother Nature’s way of forcing us to slow down and enjoy Her natural beauty! So if the wind is in your face, you’re moving uphill and sucking wind, sit up, take a look around, and revel in how fortunate you are to have the health, the wealth and the wherewithal to even be participating in a triathlon!