What is the general plan/idea in this structure?

Again, I come to this structure/position, which I have such a difficult time dealing with.

Yes, white has more space, decent development, and certainly won't get destroyed barring a silly tactical mistake.

But what to do? 

One idea is to go for c4+d5, which is the standard break hinted at. But if you notice in the position, that would require a ton of preparation. 

Anyone have any suggestions for a good, challenging way to play this as white?

This was a game started a long time ago, and there were many mistakes in the middlegame by both sides. I can analyze those big mistakes myself, but I want to know about strategical ideas before the move I indicate in the above position^. 

Are you sure , you want to stick with the Classical variation vs the Pirc defense? I mean there is nothing wrong with it, i play it my self ocassionally when i want a dull game but i know its not the most challenging one. The classical set up with h3 tends to be a positional character game where White avoids to challenge the center in most cases with an early e5 and instead invest on building developmnent. Now..you can be aggressive with e5 but is better to develop first with Bc4 and Qe2 so when the f6 knight retreat to push e6 and follow the attack, otherwise you build slowly Be3 first ( one of the reasons you play h3, is to be able play Be3) then a4 and dont rush with the light squre Bishop, since you played h3 placing him on e2 is solid enough but rather passive unless you play Qd2 and castle long. Most of the times your Bishop goes to d3 or c4 but e2 its okay.

This is a video from GM Mikhail Marin talking about how Black should deal with the positional lines in the classical set up. You can use it learning and understanding the other side.

Now if you want more aggressive ideas that put  instand pressure on Blacks camp forget the Classical set up and play the Austrian attack. I already miss the days i was reading GM Bryan Smith articles and i dont understand why chess.com let this man walk away...

https://www.chess.com/article/view/lighting-the-pirc-defense-on-fire

In your position e5 is correct but you should had follow with 7.e6 forcing black to capture and then play 8.Ng5 and Bc4 intending castles and Re1 which cause problem in Blacks developmnent.

I stopped looking at the game at move 19. This is a caro structure right, but both sides have castled kingside and the pawn is on h3.

c4-d5 is fine. Yes it takes time to prep, but your minors are still a bit cluttered. There's no immediate thematic plan to undertake right now.

Another move I'd like to play is Na4 or Na5 or a4-a5-a6 with a bishop on f3. Of course black gets to move too... but that's my basic thinking. White will try to create a pawn weakness on the queenside using his space and his two bishops. Of course some minors will be traded in the process. Just make sure the minors you have left over are active and keep trying to work on the queenside.  Now I'll look at the rest of the game.

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Ok, so after move 25 it's sort of the opposite right? Now black is doing a minority attack and it's black with the play on the queenside. I would focus on the moves before 25.dxc. What could you have done differently. You seem to respect black's d pawn and queenside pressure too much because 22.Ne4 with c3 seems to be defensive minded and then black is calling the shots. Try 22.Ne2 (now Nf6 can't trade itself to clear the bishop, and you're strengthening d4). Ne2 also makes it uncomfortable for black to play c5 because Nf4 and the d5 knight is pinned to the rook. After Ne2 now you can hit with a4 or c4. A slower plan would be something like c3 with Nd3-c5.

At that point a pretty fun game I think. Both sides trying to claim their queenside ideas are correct.

Black's minor piece setup also reminds me of a setup I like against the Blackmar Diemer gambit. Probably more boring, but I think black is more set up for 19...Nxc3 20...Bxc4 21...Nd5 22...e6 and now white's space isn't really meaningful and black has no weaknesses. In that case I'd be pretty frustrated as white if I wanted more than a draw.