KC, as you've seen, the one recurring thing here is that most of us ride regularly, and it can be commuting and not even a long commute, but this helps a great deal with our bodies being used to being on a bike.
I've always tried to do some longer rides gradually when I can, 30 or 40km rides, and like the person who mentions being on the same bike that you'll tour on, its great because this helps in showing up any slight fit issues, and over time, should allow you to identify small fit changes that make things a bit more comfortable for you.
When I commute, I often have two panniers on the bike, often full of stuff, so at least the legs are a bit used to riding a heavier bike, so this helps.
For me though, the main thing in response to your reasonable questions is to plan the first bunch of days to be very reasonable in distance.
50k 60k, so 30-40 miles, is great because especially if you give yourself time, you can take it easy, not overwork your leg muscles, have ample time to take many small breaks, lunch break etc, and get to your planned camping spot with ample time to not be rushed, so its a nice day.
Obviously, by doing rides of about this distance numerous times before hand means you know how your body will respond, so you have confidence in how you will be physically, and have a reasonable idea of what eating you need to do, proper drinking etc.
After a number of shorter days, gradually increasing distances will become natural, as you quickly get stronger and fitter.
Its all doable, just be reasonable and use common sense in preparing yourself beforehand with regular riding and gradually increased distanced training rides.