I know this is a long ago question, but I'd like to add a technician's point of view here. Heat index over 100 and I cancel walks. If they would like me to still come let the pup out for potty break, I'll certainly do that - but outdoors becomes dangerous. I would hesitate to see what the dogs energy is like and play it by ear. Old, obese and otherwise compromised animals tend to fair really poorly in heat that high. I can't tell you how many dogs come into the ER after "only being out for like a half an hour...."
http://www.humanesociety.org/animals/resources/tips/pets_safe_heat_wave.html?referrer=https://www.google.com/ (This is a really good article about heat stroke and keeping pets cool from the humane society.)
If it's hot, but bearable, take frequent stops for water breaks. There are so many collapsible bowls out there, it's easy to pick one up.
Also, great advice about keeping to dirt paths and no concrete for paws. The salve available at various pet store only works slightly. And then you have to remember to clean off paws when you get back to home, because dogs do sweat from their feet. Besides panting, it's the only biological avenue they have of cooling down.
Cheers!
I have learned from my vet do not have dogs outside if temperature is under 32 degrees and over 65 degrees their paw pads can burn and that hurts their paws a lot.