I would put more emphasis on the druidism style classes, without gods there is little to grant god-given divine magic. I wouldn't even adjust spell levels for cure spells, without the gods looking out for their creations, healing magic is more difficult. But since you don't want to do this here is another suggestion: simply re-flavor the divine magic as arcane magic being channeled a different way, so different that arcane classes (with few exceptions) cannot duplicate it (like bards getting cure spells).
Paladins, by flavor, don't necessarily need the gods. They draw their power from their convictions in law, order, justice, and THE AMERICAN WAY (cue superman music). So you could have this class without dealing too much with the issue of the world not having gods. If you need to re-flavor this, flavor it along the lines of the sorcerer from 3.5, an instinctive capability to cast "arcane" spells, but the power is fueled by their virtue rather than studied expertise or bloodline capability. This would let you keep the class and still have the spells be divine, just with a different name. A wizard seeing a paladin cast a spell would see magic being channeled in a way foreign to the wizard, as such he cannot duplicate most of those spells (Hooray for different spell lists!).
Now, clerics: there are rules for clerics who do not worship a specific god, but still gain domains. "If a cleric is not devoted to a particular deity, she still selects two domains to represent her spiritual inclinations and abilities (subject to GM approval). The restriction on alignment domains still applies."- from
the cleric page of the pathfinder OGC. With that, you could further limit this so that sub-domains are not allowed, large ideas are allowed, but without gods these cannot yet be refined to have sub-domains (once the PCs become gods your world can begin to get sub-domains). I think this may be the easiest way to handle everything mechanically. The magic could still be classified as divine, but with a different name to represent how it is arcane magic being channeled a different way than wizardry, sorcery, bardic, summoning, and witchcraft magic.
Also:
The SRD for Pathfinder You said you are new to GMing, so you may not know of this site. I find it very useful when I run a game, or play.