The temps here rarely go about 65 degrees, but I'll soon be going on a car/bike trip where temps could be near 100. So, I'm concerned with my lack of habituation to those temps.

Keep hydrated, and stay on top of your heart rate. I'd keep it slower than usual until you do get a bit acclimated. How long will you be on this trip?

95 degrees with humidity around 60% seems to be my enjoyable limit. And at those numbers training, speed, or extra distance is out the window.

114 degrees actual temperature (not heat index) in Austin yesterday and I rode in it briefly so that I could say that I did.

If you can manage the time you ride, get up early and then get off the road when your heart rates shows you're stressing too much. Gradually, I will bet you can increase the amount of time on the road as you acclimate to the conditions.

Otherwise, just ride. It's highly unlikely that the day starts out at 100 and you should have plenty of time to get in the miles way before it gets to that point.

LOL if you waited for below-80s to ride here, you won't start until mid-November. Just so I could say I did, I went for a 30-miler (flat land) the day it hit 116 here, earlier this month. Humidity was really low (15%) - I guess when it gets that hot the moisture just burns off... but it still felt awful and in retrospect it was probably a stupid thing for a 53-year-old guy to do.

We routinely ride when it's over 100 here. It's either that or get fat.

I rode last month 22 miles with 107* heat index! IT was rough,