Most campaigns don't reach their end
That's just the way it is. Doubly so for your first ever campaign. You might lose interest. So might your players. You might realize you don't know what to do with them anymore. Life might intervene. Things happen.
And that's ok. Fun would still have been had. Memories would still be formed. The world you create might become a place for a next campaign - or inspire you to make a new one. As banal as it sounds, you'll learn from your mistakes (and discover exciting ways to make different ones). Don't sweat it.
Here, then, is some general advice on running a lengthy campaign.
Break it up into acts
Knowing as you do it's likely to prematurely end, plan for that. Have several major acts that would provide some satisfying conclusion. 4e's tiers work well here, though it does depend on the pace of your game - how quickly the PCs will level.
Perhaps in the first act, the war with orcs is a straightforward affair - they invade human lands and defeating their force is the end goal. In the second act, paragon-tier PCs venture into orc lands instead, to discover how orcs have changed. They also witness humankind begin its downfall, and make a decision in the end on which side to favor. In the third act, epic-tier PCs engage with the mystical source of knowledge and civilization (God? Artefact?) and rewrite the nature of man and orc. At that point, mortal kings are beneath their concern, as they deal with cosmic forces themselves.
Don't save the cool things for later
Do you have an awesome idea? Use it now! Don't store it until the slightly more appropriate time two years from now. By then you'll either forget it, it'll become irrelevant, or you won't get there at all. Make each session fun, not a prelude for fun down the road.
Plot it out
You have an idea for what you want the campaign to be like. Use some of the tools developed by others to help you get there. 5x5 method or Dungeon World's fronts, for instance. It won't do you any good holding it all in your head - get it out on (virtual) paper.
Change it up
Those methods tend to say this, but it's worth repeating: if the game goes elsewhere - that's fine, too. Never be afraid to change your plans. Don't stick to the one true path.
Play the same game
This is what kills most campaigns - people play different games at the same table. Use The Same Page Tool to set common expectations. And always, always, always talk to your players.
So first of all, I really like this idea. I tried it once or twice a long time ago and I'm still fantasizing about it. With that out of the way, let's move to my 2 cents.
Make the NPCs well-rounded characters
These NPCs aren't just some recurring characters, not to mention some one-time ones. They're gonna be with the party for a very long time. Whole adventures, actually, which is quite a lot. This means that they should be really well rounded. Your players should get to the point of distinguishing between them after just a few words, especially when the NPCs are the ones who spark the conversation.
More than that, though, they should be more than cardstock characters. You want them to express feelings, you want them to have goals, and you want them to react. Otherwise, your players will have hard time connecting with them, and this is far less good. As a rule of thumb, make them round and distinguishable from each other. A wizard is different from a fighter, true, but we want a greater difference. The prince will have hard time adjusting to the wilderness and to the bugs in his bed. The merchant will always stop to collect the better loot so she'll be able to earn much from those bargains. Classes are a dirty way to distinguish, but they're better when accompanied by some other traits.
Don't let them speak right one after the other
The players aren't coming for the game as an audience for a theater, they're here to play, and hearing NPCs talk with each other is far less fun. If you must make them speak between themselves, make it as quickly as you can and immediately move on. If you can narrate the conversation instead ("they're talking about what happened, Elsa thinks that they should go east and Hans thinks that they should go west…"), it is far far better.
Give the players the center stage
The players and their characters are the real stars of the campaign. The NPCs are extras, and should always be seen like for you that when you're playing them. If they're far cooler than the PCs, or if they have much more screen time, something is off with your campaign. It's better to not have the NPCs with the group than to let them illuminate and shadow the PCs.
They're not all knowing
Think for yourself what is more important, a character who knows everything or a character who knows only part of them. For me, it is the latter. That comes from one simple thing: There's no drama when everyone knows everything (it is not entirely true, but that's for another time). Make them say sometimes stupid or idiotic things, make them come to wrong conclusions, let them make mistakes. They're not a kind of supernatural deity who knows everything, but humanoids who are as humane as the characters, and they should be played this way.
Make them important for the story
They should always be important to the overall story, in one way or another. In one of my more successful D&D campaigns, the characters had to escort a princess to a neighboring kingdom through the forests. Having to keep her safe from one hand, and dealing with all of her complaints from the other one made the game so much richer. They don't have to be important to each and every one of the scenes, but they should always be important for the overall story. Otherwise, the characters may just leave them to rot one day, when things will turn the wrong way.
And an end
Hope I succeeded with helping you a little bit.
Best Answer
While it is true that 4e does center around combat, not all conflicts are about combat.
If you find some good resources on RPG plots, like S. John Ross' List of RPG plots, it can give you some ideas about running exploration adventures. Especially what he calls the Safari, Any Old Port in a Storm, and Uncharted Waters plots are good starting points to work from, as you want a focus on exploration. As for the implementation of this plots, remember your skill and social encounters!
The social encounter and the skill encounter will be your best friends. Social encounters are good for talking to "natives," maybe even talking to other random explorers. Skill encounters would be good for general scouting, climbing a mountain, opening a lost tomb, tracking a gryphon to its nest, and so forth.
Of course, you can always find a fight in the wilderness. Maybe the adventurers ignored the obvious signs of an owlbear's territory? Perhaps a random band of orcs or gnolls are inhabiting that ruin they discovered a few days ago?
As for generating the wilderness, I'm afraid using LOTR as a spiritual guide may be best. It also depends on what your setting is. If the adventurers are in a well-documented world, like those WOTC produces, then you could use those as a guide. If it's a home-made setting, then it's up to you how to populate that world. (Again, looking at a the WOTC campaign settings can be helpful in making your own.)
NOTICE: Subjective Input follows! I would recommend rough maps of places for adventurers to go to. This will give you an idea of what can happen in the coming session, and will likely produce a more cohesive wilderness rather than a nonsensical generated one. With such a planned wilderness, you can even have past exploration sites affect others. This means the ruins over there give a clue about the door to the ruins over here, or something along those lines.