I'm new to touring too, only did two tours so far this summer but I think I learned enough that I can contribute some blah, blah, blah

It depends on what your goal is: are you riding because you want to get to some particular place or because you want to enjoy the riding itself and your destination is just an arbitrary ending of your trip?

Touring is about enjoyment of the ride and the surroundings IMHO. The destination is your secondary consideration. I don't think you can enjoy it if you're exhausted every day. Even for a very fit, young person 100+ miles a day seems unreasonable for anything more than 3 days in a row, for a tour that is. I did 50-60 miles a day in the Adirondack mountains for 10 days and felt fine. Then I did, by accident and poor planning, two 80 mile days in Massachusetts and Connecticut and that burnt me out and I hurt my Achilles tendon and my knee started to hurt. I'm not very fit by any means so I consider 60 miles per day the most I can do unless it was flat and the weather was cool and dry. I think you can reasonably expect to be able to do perhaps 80 miles a day, but not more than that.

I would carry at least a full day worth of backup meals, if not two days: dehydrated meals, ramen, couscous, canned chicken, that kind of stuff. Just in case you find yourself in a very rural area. I rode about 20 miles in MA without seeing a single grocery or convenience store or a gas station. I expect some southern states may have even more scarce resources. So don't count on being able to stop and get a meal whenever you want and small snacks may not provide enough nutrition. But I guess being a long distance runner you know a thing or two about nutrition

You will likely spend more on food on tour comparing to being at home unless you set yourself a strict daily budged and be very mindful of your spending. Why? Because you're likely to eat more, snack more and may not always have the choice to buy your food at the cheapest places. Although, for me since I live in NYC, I found that all the places I rode through were much cheaper.