What I learned on summer vacation:
Ride Across Indiana (RAIN) in 2019 kicked my tail, heat index 100-105 from 10 am to 7 pm or later, and the ride is 165 miles long.
Ride Across Wisconsin (RAW) in 2019 found me bonking at 183 miles. Cramps and generally bad.
Before 2021: 1 century a year, maybe. 2-3 metrics, and when I rode in 90+ degrees, I took 2 water bottles and limited myself to 55 miles or so.
I knew what didn't work: alcohol consumption, water only, not pacing, and too long at rest stops.
As a former consistently 2:30-2:40 marathoner, I only drank water. I thought I was good. NOT.
2021: 16 centuries in 11 states, including Hotter'N Hell Hundred in Texas, a 228-miler (RAW) and training rides from 50 to 150 miles, in the heat of the summer.
When planning 2021, we asked advice from 3 cyclists, all over 60 or pushing 60, who were stronger than us.
They'd had heart issues, cancer, etc. Tough as nails. Myself: 1 minor heart attack and a minor stroke.
They were all about:
1-Consistent daily hydration: water, tea, non-sugary stuff. None drank sodas or sugary drinks.
2-Moderate alcohol at the most, moderate caffeine at the most.
3-Preparation and staying ahead of the hydration curve on rides.
We formed a group and planned for our 228-miler by riding in the heat all summer, establishing a routine and getting to know each other's style and being able to spot any issues. We also practiced 2-minute pulls in a pace line, ride etiquette, and communicating. In the heat, good habits help a lot.
Pre-ride hydration:
No alcohol within 2 days.
1 hour before: a concentrated hydration drink. Some used Hammer products only, some used other brands. I used Skratch Hyper (sea-water!).
Ride hydration:
First two bottles, sort of keep the prep hydration going. I used 50/50 Beta Red and Skratch (regular). Stay ahead of the curve. Others used their preferred brands.
Every bottle thereafter included a product, be it Nuun, Hammer, Fizz, DripDrop (which we all liked the best). I've since switched to Ultimate.
10-mile maximum gaps between "cheers!" reminders. 1 bottle per 20 miles became about the standard, and we timed stops to find stores, etc with ice/water. Carried our products.
Recovery hydration:
Low-fat chocolate milk or ice tea with honey.
We did drink alcohol after, but that buzz came cheap.
We simply avoided sugar pretty much all the time, very moderate on caffeine and alcohol.
We did several centuries above 90 degrees, one above 100. We were fine. We did a 100/100 3 weeks before the double, and 150 2 weeks before, both weekend in the upper 90's. The hydration scheme worked well. Each was left to their own on clothing, caps, sleeves, etc, but the sun is not your friend for those kind of miles. I switched from Chamois Butter to Diznutz for it's ability to withstand sweat, and I switched from padded saddles to straight carbon for the ability to move around. Others did what was comfortable for them.
Results: No cramping, no bonking across the group. Plenty of PR's in the group on 100's and even metrics, Climbing PR's, etc etc.
Exception: don't give blood on Tuesday and do a 8,000' century on Saturday, or follow that with a 9,000' century in 90-degrees the next Saturday.
The mileage paid off. One member didn't want to do more than a couple 100-milers before the 228. He didn't do the back-back weekend or the 150, preferring a different regimen. He weakened considerably at about 165, but due to his hydration routine, actually recovered well by 185 and we were good to the end. His hydration helped him overcome his mileage deficiency.
Hotter'n Hell Hundred: Easy-peasey, and really, it only got to 100 during the last half, just the 90's in the first half.
All I can say is that it was a great time from May to September, and consistently planning and carrying out the routine made it doable.
Your results may vary, but when I go out now, over 50 miles, I pre-hydrate and keep at it.