I never used to let Biscuit go on walks during the day when it got really hot here. He could handle cold --- loved it, in fact --- but didn't do the best with heat. You might try taking the walks really early morning or at night.
Water water water!
I also think some dogs are okay with heat, and others aren't. And it doesn't seem really predictable. Yogi, for example, has that underlayering of fat that labradors have --- you'd think he'd hate heat, but he loves it. The hotter the better --- he gets out and roasts for hours and hours at a time, seriously, in full sun, and will not come in. I have to drag him in because I am just so sure he's going to collapse from heat exhaustion. :roll: But he's always just fine. ?