>>> ... think we can manage getting fit ON TOUR?
Short answer: Yes. Just wanted to add my experience to the list.
In 2001 my wife and I rode 2000 miles, from Oregon to Yellowstone and back, in 6 weeks. I commute on my bike every day -- between the two of us, that was the only training we did. It was a loaded, self-contained tour, so we knew what we were getting into, and we planned accordingly.
The first 4 days were tough, increasing gradually from 25 to 60 miles. Mostly sore neck muscles, sore shoulders, sore backside. And quick lessons learned in keeping hydrated and fueled -- looking back on it, that's one mistake I won't make again, most of the physical discomfort was from not drinking and eating enough. If you have to, force yourself in the first few days.
By the 5th day, we both felt we were in transition. More energy, less complaints
By the 6th or 7th day we were in the Rocky Mountains, and we were in the groove -- I'd say we were 95% of what we had achieved by the end of the 6 weeks. A lot of that has to do with adapting into your routine: handling traffic, communication on the bikes, setting up and breaking camp, knowing when either one of us needs ice cream. When you're forced to adapt to a strange routine, even small physical discomforts loom big. I discover this revelation every time I tour: the "training" needed comes in the first week, when the tent/spokes/pedals/food/water/maps/gears/bakeries/climbs and descents all meld into one and you don't want to live any other way.
So in my book, training 'per se' isn't necessary for a good tour. The biggest exception is when you're in a group of disparate abilities or fitness. Then, the least fit will suffer. As long as both of you are healthy, you know your bikes, and you've practiced a couple of times (you did good with the shakedown rides!), you'll be good. Headwinds will grind, ascents will turn your legs into rubber, heat will suck the life out of you (yeah, we survived!!), but it's your attitude and mental preparation that get you through. Your planning already shows that.
-- Mark