Be a bit more realistic in goals.
12,000 ft. of climbing? 100+ miles? Be prepared to be picked up before mile 50!
The hardest ride we've done (on a tandem) was a 325 mile ride with 22,000+ ft of elevation gain in Arizona over 3 days.
We were then in our late 50s and in excellent condition.
Tough enough that my wife/stoker said she would not repeat it, ever!
Have ridden in Utah, and yes, there's some serious hillwork on that course.
Wait an extra year (and drop another 100 lbs) see how you feel then.