Training Rides

I have signed up for a ride in Colorado coming up in June. 62 miles in the mountains in Castle Rock, CO.

I have never ridden in the mountains so I am looking for a way to simulate the mountains and lack of oxygen. The ride we are doing is 62 miles and starts out at 6200 ft above sea level and up to 7500 (close)

There is one section that is 12 miles of uphill climb. I am pretty sure the ride will be doable but still kinda scared. I am taking my children to visit my brother in law, Ryan. Ryan asked me about signing up for the ride and my wife said ok. I am convinced she approved only because her Dad and brother were jazzed about it. The kids will not ride but are going to do some cool tourist stuff while in Colorado Springs.
I am training with a new training partner who rides really well. Lloyd races most weekends and I am recovering from a hard Friday ride. Gardening and an easy spin ride yesterday left me pretty wiped out. Got a little sun when it was warm yesterday but now it is really cold.