It is only an hour from Santiago. You can get there:

By Bus:
There is a bus service which leaves from the Terminal San Borja in Estación Central every half hour. The journey takes 2 hours.

By Car:
To drive to the Cajón del Maipo from central Santiago you can either take Avenida La Florida, or go on Avenida Vicuña Mackenna as far as Puente Alto, then turn left on Calle Eyzaguirre towards Las Vizcachas where you will get onto Ruta G-25 which will take you up the Cajón. Ruta G-25 is paved as far as the village of San Gabriel at kilometer 56.