Tara, forgive me if this comes across as patronising but there's a whole lot more to cycle touring than distance.
Working from the assumption that you haven't ridden seriously in 20 years, 100 mile days in a row is a big ask!
At an average of 12mph that's 8 hours straight on the bike. Start at 8 am and cycle continously until 4pm. Stop for lunch? Then it'll be 5 pm. Take a few breaks then arrival is back to 6 pm. Running behind schedule? That just adds to the pressure. I don't like touring under pressure.
(For the sake of comparison, I rearely clock up more than 5 hours moving in a day).
Assuming you have a bike, my advice would be to use it as much as possible. Get to know how your body reacts. What does 50 miles feel like? And the next day?
When you have an idea what speed/distance you're comfortable covering, repeatedly, then you'll be in a better position to plan.
I've seen it said before that for a regular cyclist if they add up their weekly mileage then that is roughly the distance they can expect to cover in one day. From my experince it seems to hold up - I commuted 200km a week and would be confident of being able to cover 200km in a day. Of course, touring, with baggage and presumably an emphasis on enjoyment, will be a bit different.
For me, touring is all about tbe experience, being in the moment. There's a wonderful sense of freedom, there's no work, no appointments. Being able to stop and appreciate wherever I am and interact with the people I meet. Food, no matter how basic, can taste delicious! These are all difficult things to experience and appreciate if we're rushing, racing the clock and tired.
For what it's worth, I have never trained for a tour. I do practice, though. I go away for long weekends or little overnights. I prefer to camp. I'd often cycle a roundabout route to end up in a campsite 10km from my front door. In winter, I practiced cold weather camping on a friend's farm. I learned about navigation, eating, drinking, cooking, gear, weather, mechanics etc. by doing. I was getting fitter, sure, but I was also developing knowledge and skills that made a bigger tour far less daunting and more enjoyable.
Best of luck!
