Schemas of history are not unique to religion. For example, one popular schema divides history according to whether the years in question precede or follow the Age of Enlightenment (roughly 1650-1800). We hear of Ages of Stone, Bronze, and Iron from historians and of Triassic, Jurassic, or Cretaceous from paleontologists. The most famous chronological distinction is perhaps BC/AD, which is explicitly religious -- though there is also AH, which counts years from Mohammad's flight to Medina.
The term dispensation has long been used in Christian speech to denote not only the period in God's economy belonging to the Incarnation, but other distinctions based upon the Fall of Man, the Flood, the giving of the Law to Moses, and so forth. It is not unique, as might be thought, to those versions of Protestantism that call themselves Dispensationalist -- who have, however, earned the sobriquet by their taking such schematizations to baroque levels of detail and controversial levels of doctrinal importance.
Common to many schemas of history is the notion of a Golden Age. In Buddhism, for example, the Siddhartha Gautama's life was a Golden Age, followed by two ages in which his Dharma could thrive. A third, much longer age -- the age we are currently in -- is mo-fa or mappo: the Degenerate Age of the Dharma.
I suggest using the idea of dispensations such as the Degenerate Age of the Dharma in the sub-creative act. When turning your mind to mythopoesis, try this question: What are the events that define the history of this world? You can ask this question both from a normative point-of-view (What are the events that really define the history of this world?) and from a descriptive point-of-view (What are the events that define the history of this world from within the point-of-view of a given culture?). Defining the ages of your world will give you a history to work within, and describing them from the point-of-view of cultures within your world will help create those distinctive cultures, religions, and so forth.