I live in SC Nebraska, so altitude and long climbs are challenging to train for. The best advice I got was to put a
Walmart bag over my head and find any hill to climb. I didn't take it, although it might have helped. My first TBP was last year. I attempted the Double Triple. Completed the first day and 90 miles of the second. Bonked out going up Loveland and didn't recover even after the long downhill to Idaho Springs. For me, keeping my weight down and doing a butt load of intervals is key. And starting early. Acclimating is a no go for me. Unless I could be out there for at least a week. I'm best to hit it right off the bat. Otherwise I progressively get worse with the altitude with each passing day. Surprisingly some of the supplements containing Rhodiola Rosea have seemed to help with endurance. At least for me. I think it's important to do a few rides of that length or more as a part of your training. And don't forget to enjoy the landscape! Good luck!