A hot, dry heat is more tolerable than a hot, humid heat. This is referred to as the heat index. Indeed, during very humid conditions the sweat on our skin is unable to evaporate and ceases to provide an avenue for body cooling. These points are by no means exhaustive, but I just thought I would fire a few out there. This not only refers to the question of whether you have been living in a hot environment, but have you actually been exercising in the hot weather.
Plan runs that have regular opportunities for water stops.
For remote runs, you may want to drive by the route on the way home from work and stash a water bottle in some bushes for when you run past. Clothing is also important. For dark skinned individuals, it is better to wear a light coloured singlet or T-shirt than nothing at all. Loose fitting clothes also help prevent hot air from being blocked against the skin. Finally, i think it is important to consider the relationship between the temperature and the humidity.