I live in central FL. I have ridden tours in the Rockies at elevations over 12,000' (Trail Ridge Road in Rocky Mtn Natl Park). I have never had a problem with exercising and elevation.
Response to altitude varies markedly from person to person. Some don't feel it, a very few get High Altitude Pulmonary Edema (HAPE) or High Altitude Cerebral Edema (HACE) - both potentially fatal.
Most likely, one will get a headache and shortness of breath. DO NOT DRINK ALCOHOL. Keep very hydrated, rested, and get to CO as far in advance as possible. It actually takes several weeks to really adjust to altitude.
Folks from Florida and other low states complete the RTR successfully every year with no particular problems, especially if they are in great shape to begin with.
Train as much as you can, especially on hills and the like.
Keep in mind that Colorado is extremely dry. Even in thunderstorms, it is dry between the drops. Be prepared for snow and sleet - it can occur at any time during the summer.
Mostly, have fun!!