100 mile days are fairly easy to knock off once you get cranking on a trip. It takes me a week or so of touring to get in proper shape. I have gotten in a little trouble occasionally - exhaustion, cramps or just plain feeling lousy - pushing it too hard right off from my home. If you run out of gas just take a rest day or a short day. Allow for it on your schedule.
Like a horse smelling the barn I have cranked out fairly high mileage coming home from a long tour out west. I once did the 667 miles from Chattanooga TN to my home in Florida in six days with ease - arrived home feeling fine, even rested. I once made it home from San Antonio TX in 15 days in the short days of winter. I live in Holiday - 30 miles north of St Petersburg.
Like you I can take the heat. I've lived in Florida for 39 years without air conditioning. But I will admit 114 degree weather in the Badlands of South Dakota once had me doing a short day.
I used to make the 430 miles up to my brother's place in Savannah GA in four days. At 74 years old I now allow 5 or 6 days but I still make the return trip home in four days easily.
Bicycling is a forgiving sport - non weight bearing and easy on the joints. You are basically just using a couple of big muscles in the upper legs. Once you build them up you can crank out mileage the non-cyclist finds incredible.