Actually the Lord of the Rings offers a great example of how PoL works as a campaign setting.
In the begining there is only darkness. The elves make small points of light when they appear and the evil god Melkor attackes them, tricking them into thinking it is Orome's fault.
(Campaign: Begin in village, have adventures, retire at level 30 in Valinor)
The elves do wondrous things in Valinor and eventually through his agents Melkor is both freed from his prison and destroys the Light - plunging the world into Darkness. The Noldor go back to Middle Earth to overthrow Melkor and reclaim the Silmarils.
(Campaign: Begin in a village, have adventures (maybe even encounter Sauron, or Ungoliant). The heroes either retire to Valinor or are involved in founding Numenor.)
Numenor rises and forms an Empire (of which Gondor is a part). The Rings of Power are forged and Sauron forges the One Ring, Hollin and Moria are cast down/into evil in war with Sauron. Sauron eventually appears (in fair guise) and corrupts the kings of Numenor, and causes it to fall when they attempt to sail to (and attack) Valinor.
(Evil Campaign: The PCs work for Sauron to help him undo all the good of their last Campaign. The PCs retire to Mordor or Middle Earth...possibly becoming the Nazgul later due to their Magic Rings (Epic Destiny for Evil guys..sometimes being immortal is a bad thing)
)
Mordor rises as an Evil power against the failing light of Numenor (now only the domains of Gondor). The Last Alliance of Elves and Men is crafted and the remnants of Numenor and the Elves assault Mordor and Sauron is defeated, but not destroyed.
(Campaign: The PCs start in a village and eventually become involved in the War, and do things like fight and drive off their now Nazgul former PCs. The PCs retire to important positions in the now safe Gondolin or one of the Elven nations (or if elves go to Valinor).)
The One Ring is lost. For many hundreds of years nothing bad really happens. Eventually the Necromancer rises in Mirkwood, and evil things bestire in the land. Eventually White Council attacks the Necromancer's fortress of Dol Goldur, but he flees before them rather than be killed, for he (being Sauron) knows the One Ring still exists and he hopes to be re-united with it.
(Campaign: PCs might live among the men that are moving into the Misty Mountains, spend time fighting goblins and such until eventually they are involved in either the Battle of Five Armies, or the assault on the Necromancer. Either way they retire into positions of importance. Campaign ends in high Paragon low Epic Tier depending on the Epic Destinies available.)
Roughly 80 years pass in which nothing happens of note. In the following 20 years Sauron returns to Mordor and begins gathering his forces. Roughly 100 years after fleeing Mirkwood he learns the location of the One Ring and sends the Nazgul to retrieve it. Around this time Gandalf determines the nature of Bilbo/Frodo's Ring, and Saruman and Denethor are corrupted through their Palantirs. Eventually the Sword that was Broken is reforged, Aargorn defeats the army of Sauron that besieges Minas Tirith, the Witch King is killed by Eowyn. Aragorn leads his army to the Gates of Mordor, and while the battle rages Gollum, driven by Frodo's curse falls into Mount Doom and the One Ring is destroyed.
(Campaign: The PCs would probably be from Rohan, making them involved the stopping the pillaging of the Westfold, the Battle of Helm's Deep, the Battle of Pelennor Fields, and finally the battle at the gates of Mordor, they might kill a Nazgul before Gollum dies with the Ring. PCs retire as heroes of Gondor/Rohan to positions of power.)
Its only significantly after this in the latter days of Aragorn's reign (which last's hundreds of years) that "Middle Earth" is really not a PoL setting, and even it has an "End of Days" scenario with the return of Melkor and his resurection of Sauron and so on.
That still takes at least 6 Campaigns, some of them would be "Legacy" campaigns some would just be seperate campaigns. That's a lot of mileage and thats for a far more coherent setting than the PoL setting - there is a lot less grey in Middle Earth than what we know of PoL.
A more "DnD" campaign to look at is the Savage Tide campaign from Dungeon - it even ends with the possibility of a PC achieving an Epic Destiny. Currently the Campaign is spaced over 20 levels but it wouldn't be hard to adjust it to go over 30 levels really.