No reason why the heat should be too much of a problem, take plenty of liquids. I freeze it the night before so I have a steady flow of cold water.
Wear appropriate clothing, carry sun cream, drink lots.
Most of the guiding companies will go out during the cooler parts of the day and should be well used to dealing with people in the heat. So unless you are one of these people who doesn't like hot weather, I wouldn't worry.
That is said by someone who'll ride in the 30âˆ«c-35âˆ«C of a Madrid summer.