Ride lots. I'm over 70 and still tour and ride hard - well hard as I can, which is as hard as ever, just slower
. It's the same for any age. You have to get in shape by riding a lot, get experience in being on the road, that sort of thing. Along the way, all these questions will be answered for you.
What you do to start with is once a week, ride away from home until you're tired. Then ride back. The other days just ride for an hour or so or take a rest day from it. Keep track of your weekly mileage. Increase your weekly mileage by 5-10% each week - or what you can, but try to gradually do more. Adopt a "see hill, ride up it" philosophy. In spite of what the average person might think, cycling is all about riding up hills. A touring cyclist needs to get really good at that, which takes time, at least a couple years of consistent riding. When you've been riding over 100 miles/week for a few weeks you'll know a lot more about it.
Saddles and bike fit are critical elements. If you already have the bike you want to use for touring, go get a bike fit and a saddle recommendation from the fitter.
In order of importance IMO:
Clothing - learning how to dress for any likely weather and be comfortable in it..
Fueling and hydration - learning how and how much to eat and drink on long rides.
Shoes and pedals - IME it's easier to ride and tour long distances with SPD clipless pedals and MTB shoes
Bike - I'll put that last because you can tour on any bike to which you can attach a rack and panniers.
That's all a tall order and will take you some time to figure out. As it is said, experience starts when you begin.