I'm having the same thing with my dogs so haven't taken them fro a run today - they are just sloping around the back half acre, lying down, getting up, walking a bit, having alp of water, lying down again - let's just hope we get enough of this heat for them to get used to it. Me? I'm used to it now - bring it on.
Seriously, though, if they have space to get away from the sun, shaded areas, plenty water, that's all you can do except maybe turn a hosepipe spray on them if you don't have a ban where you are