Bob,
I rode this race 3 years ago and then the Newton Revenge 2 years ago. My training was 100% on indoor rollers and I don't think I rode more than an hour a day and no where near 3400 miles that year. I'm three years older than you (now 55), so I rode it the first time when I was your age. My time was 1:13 the first year and 1:15 the second year, so turned out the training was at least ok, and when I thought about it, riding indoors is perfect because it is 100% pressure on your pedals for the entirety of the ride, which is what Mt. Washington is all about. It's a power ride, so train for power. Maybe find a gym where you can really work on sled pushing or some other explosive quad power. Really helped me.
Here's what I would recommend:
Get in your head the idea of a road that never flattens out, regardless of what you think it will look like at the next turn. In other words, visualize a road that is very steep and will be steeper around the corner. You will be mentally challenged otherwise.
Just keep pedaling. Don't think you can save your gearing. You'll be in your lowest gear from the very beginning. After the initial "downhill" (a drop of about 10 feet in the first 500 feet of the race), everything and I mean everything is between 5% and 21% grade. Mostly in the 12% to 17% range I think. Definitely get the right setup though--talk to your bike shop about that one.
Don't worry about your time. The end will come. Just keep pedaling. And you can probably ride harder than you think you can, so if you want to improve on that goal time, just ride a little harder. You'll be riding hard anyway so just add a little urgency and it will work.
I drank Ironman Endurance. Hard to get a gu open, never mind down, when you are pedaling uphill the whole time.
dress for it. The first time I rode it was hot all the way up. The second time it was warm at the bottom and then in the 40s with 40+ mile wind gusts at the top.
You can do it. There are people riding tandems and others riding unicyles. I don't know how they do that, but it means you can do it.
My own view is don't ride the practice ride. I didn't because I thought that was like jumping out of an airplane to practice getting ready to jump out of an airplane. Just enjoy the race. Also, riding the practice ride will create expectations that you feel like you'll have to match during the race and for lots of reasons (weather, primarily) the days may not be the same. Just ride with no expectation and the knowledge that you're supposed to hurt but it will end soon and will be awesome, and the worse the weather the more epic the story.
Put a camera in your car and a LOT of layers for the top. It is an awesome experience It can be really cold up there and you'll be there for a while until they open the road for the way down (when everyone finishes)/
Have a great time!