A couple of points you should consider. Firstly, if you have never ridden before, you probably don't want a full day!! An hour or hour and a half would probably be enough for first timers. Especially if you want to be able to walk/sit/ move in any way afterwards!! Second, it is really hot for riding at the moment, and will be in early August. You should consider going early morning or late evening, and preferably somewhere that can offer a certain degree of shade for most of the ride i.e. through woods/forest.
I have access to 20 horses and ride every day, but consider very carefully what times of the day I ride at this time of year.
I went to this place a couple of years ago - www.horseridingmarbella.com - and they offered a good service at a reasonable price.
www.lahaciendacasares.com