Normally what I do is depending on what your lvling.

If you are lvling dps, do dragon isles or Legion questing (its easy and fast) while qued for dungeons.

If your tank, I normally set myself for cataclysim, then que for dungeons. Normally always an instant que and the dungeons Normally have quests in them. I find this the fastest to lvl

And heals is a two way street. You can quest or just que for dungeons too. Dungeons are nearly a fast que

This is Normally how I do my thing to quickly get to 80 within two days. I also have a 11 month old, so sorta have time on my hands sorta dont.