Biking 10 miles when the heat index is over 100, along my usual mostly-flat, shady route: Mediocre idea. Not as fun as in nice weather, but fine.
Biking 10 miles when the heat index is over 100, along the highway*, which weaves up and down along its entire length and has zero shade: Bad idea.
Biking 10 miles when the heat index is over 100, along the highway, after having biked 19 miles already: Worse idea.
Biking 10 miles when the heat index is over 100, along the highway, after having biked 19 miles already, after packing my water under a bunch of other stuff so I can't get to it without unpacking: Worst Idea.
I refilled my water bottle before I left Panera, but made the mistake of putting my frozen mocha in the drink holder, while my water bottle (which is too large for the drink holder) got buried under all the other stuff jammed into my overcrowded basket for the ride home. Even so, I was fine until I hit the first and steepest hill, along a shade-deprived sidewalk.
My body [reaching the top of the hill]: WATER.
Me [takes a drink of no-longer-frozen-but-still-cool mocha]
My body: THAT IS NOT WATER.
Me: Oh, come on. It's ... mostly water. It's liquid! It's got to be at least ... sixty or seventy percent water.
Body: DIE IN A FIRE. Which the sun is conveniently arranging for you, by the way.
Me [bakes on the heat radiating up from the blacktop and down from the sun]: I'll stop and unpack the water when it's convenient. It's only like ... three more miles until we're off the highway. 20 minutes tops!
Body: I HATE YOU SO MUCH.
Body [a mile later, while going up a very long slope]: WATER NOW.
Me: Let's just get up this hill.
My body: NOW.
Me: It's not ... much ... farther.
My body: NEW PLAN. STOP NOW.
Me: Okay, okay. [Pulls over at gas station/convenience store]. I'll just ... get the ... water out.
Body: A/C NOW.
Me: [tugs ineffectually at solidly-packed basket]
Body: Look, you can go inside NOW or take your chances with fainting on the sidewalk. Maybe the nice man in the medical transport van will take pity on you.
Me: [Goes inside. Does not lock bike. Does not take basket. Stands by door for 1.5 seconds before deciding sitting on the floor is fine. Convenience store clerk peers around stacks to stare at me.] I just need to [pants] sit for a minute or two out of the heat. [pants] I'll be fine.
After a few minutes, I went back out, dug out the water, made sure it was positioned so I could reach it while biking this time, and continued on. I did stop a mile later at a restaurant to sit in A/C for another couple of minutes. Biking in full sun on blacktop in 100+ heat is brutal. But once I made it off the highway and onto shaded side streets, I was fine.
If I do that again, though, I am going to dig out the soakable cloth wrap I got at an SCA event several years ago first. Yeesh.
Still, it was a pretty good ride for the first 19 miles. (It was 10-15 degrees cooler for the ride out along the highway, because the day hadn't reached peak temperature yet.)
* This is not an interstate -- the "highway" in question is a four-lane road with lots of traffic lights and a speed limit of 40 mph.