Many of our previous dogs struggled with heat in the 70's and above. Some of that depended on whether or not they were in direct sun.
Here in the desert we try to stick to shaded areas when walking, and at this time of year the only good time to walk is before or right at sunrise. As soon as the sun pops over the horizon it is just about too hot here to walk for the rest of the day. Rocket always wanted his walk regardless of weather, so we would load him in the van and drive to a tree lined street so that we could walk in continuous shade. He got 2 rides and a walk that way and he was happy.
When we've had dogs that overheated, we usually use cold wet towels and washcloths on their necks, belly and paws while aiming a fan at them on high. You do have to constantly change the towels or washcloths out as they do no good one they warm up against the skin. We keep a small fan ready to go and sit it right on the floor in front of them when this happens.
At this time of year, we really don't do many walks here as the dogs start panting as soon as we are in the sun. Mark, who is a black dog, will go and lay on the back yard in the hot sun at least twice a day. We do limit his time and make him come inside, but he has little interest in going for a walk in this weather, but likes to go out and warm up a little since our AC won't shut off again until November.
Good luck!