Two years is a long time to plan ahead for a tour. We just decided to go and went in a few weeks.

I would say just ride and generally stay in shape. Three of us did the Trans America (4344 miles) in 10 weeks without any real training. We averaged less than your goal mileage at 60 miles per day though. I had been riding a good bit leading up to the tour, but my two companions only had a handful of moderate length training rides in the few weeks before the tour. I think their longest pre-tour ride was only 33 miles since they were very busy finishing their senior year of college.

We just took it easy for the first two weeks or so and rode ourselves into shape on the tour. That worked out fine for us. That said you will find it easier if you have some more substantial mileage in before the tour.

Some shorter tours as dry runs may be helpful, but we didn't do any and didn't regret it.

If you have some experience with some kind of self supported travel like backpacking or canoe/kayak camping and are generally a resourceful person you will find the camping/cooking part pretty easy. Otherwise at a minimum be sure you know how to use your camping/cooking gear.