Midgame Ideas in the Caro-Kann Most of the writing I've found on the Caro-Kann online simply makes vague assertions about how "Black's endgame is better" or how White should try [some dubious move or other] to get a good attack in club play.

Is that all there is to this opening? I've been trying it out, and I find myself unsure of how to proceed in the midgame, on either side of the table. As Black, should I really just try to trade down to an endgame immediately? As White, have I no alternative but to try something speculative to keep Black from getting to his desired ending position?

I see the Advance Variation (2.d4 d5 3.e5) most often, so let's restrict the conversation to positions arising from that line.

[FEN "rnbqkbnr/pp2pppp/2p5/3pP3/3P4/8/PPP2PPP/RNBQKBNR b KQkq - 0 3"]
Most of the writing I've found on the Caro-Kann online simply makes
  vague assertions about how "Black's endgame is better" or how White
  should try [some dubious move or other] to get a good attack in club
  play.


That is not entirely true. This is the case when White overextends himself without any compensation which occurs only if he misplayed the position. In normal case both sides have good chances in an endgame.


  Is that all there is to this opening? 


Definitely not! 


  I've been trying it out, and I
  find myself unsure of how to proceed in the midgame, on either side of
  the table. 


Caro-Kann has lots of typical middlegame positions that require creative thinking and good positional play from both sides in order to reach decent endgame. 

Not knowing them "makes your life unbearable" and you will most of the time find your opponent crushing you seemingly effortlessly. Therefore you must learn typical middlegame plans for both sides. Only then, you can start thinking about endgame. And for the record, the endgame holds chances for both sides. Yes, White often overextends with pawns but gets active pieces, especially king, in return which compensates and sometimes matters more than a healthy pawn structure. 

If you need help with these post a new question and notify me via comment. I will try to help.


  As Black, should I really just try to trade down to an
  endgame immediately? 


No! Just trading down will not help. Who can guarantee that your opponent will accept the trade? 

Against such policy I deploy a neat trick:

As soon as I notice that my opponent starts to offer exchanges too often I lure him into a won endgame for me. In a nutshell, the point is to accept the exchanges that are favorable for you as much as you can so he thinks that this will turn into an easy draw. Once he offers bad piece for your good one you just decline the exchange and now you are left with a better piece against worse one which results in an easy win. 

This is a standard approach strong players use. Chess would be a poor game if equality could be reached by simply exchanging. 

As Black, your main goals are to exchange 2 minor pieces and neutralize opponent's space advantage by performing c5 or e5 break. Those are the exchanges you seek in order to cripple White's attacking potential. Prefer to exchange bishops for bishops and try to leave the knights on. After that you need to evaluate when is suitable for you to enter endgame and when you can stay in the middlegame and start attacking opponent's weaknesses.


  As White, have I no alternative but to try
  something speculative to keep Black from getting to his desired ending
  position?


As White, you can steer the game to a better endgame for you as most strong players do. 

It is highly unlikely that your attack will end with a mate against skillful defense but if you maintain pressure you can transpose into endgame where your pieces are more active which should give you a small but lasting advantage. Also, in most lines White has 3 vs 2 queenside pawn majority which ensures better pawn endgame for him. Active pieces coupled with this pawn majority usually win the game as Black needs high technique and precise play in order to hold this type of endgame. The benefit of this approach is that White can not lose the game, but Black usually draws with correct play.


  I see the Advance Variation (2.d4 d5 3.e5) most often, so let's
  restrict the conversation to positions arising from that line.


This variation sums up my previous statements very well. 

First off, middlegames can be ultra sharp if White chooses Nunn-Shirov attack and "the last man standing" will have a won endgame most of the time. In this line tactical awareness and combinational skill are important. This is the line your resources referred to. If Black defends successfully the endgame is usually won for him. However, White can create such a strong pressure and choose to enter a better endgame. 

Endgames where activity count the most involve rook and knight endgames. Especially in knight endgames space advantage is important.

Second, it can also become positional encounter if White chooses Short line and simply try to maneuver "around Bf5" in which case a complex struggle awaits both of you. Creativity and positional skill is important in that line. Both sides need to be carefull when entering endgame.

Since this topic is to broad to be covered here, I will provide brief explanations and recommend resources for studying at the very end of my post.

NUN-SHIROV ATTACK

White has 2 plans here:


Playing Nf4 at some point, planning a sacrifice on d5. The usual setup is O-O-O + Qe2 + Bg2.
White plays Nd4 preparing f4 and f5. This one is considered more dangerous than the above plan.


Both lines lead to very sharp struggle and one mistake on any side can lose the game.

SHORT LINE

One of the most dangerous tries for White. He simply plays naturally, organizing his pieces to exploit c5 break. If Black plays too slow he will be suffocated, yet if he opens the game White will have an advantage since he is better positioned and better developed. Still, Black players were able to find a way to reach full equality by preparing f6 instead of c5.



CONCLUSION:

Caro-Kann defense is a passive opening for Black that requires patient play in order to equalize. Black reaches full equality over time. White needs to do something with his greater space and aggressive pieces if wishes to win. He can opt for ultra sharp lines or play positionally, aiming for favorable endgame or simply smothering Black.

This is a vast topic to cover here but these books helped me when I needed to learn to play the Black side:

Joe Gallagher-Starting Out the Caro-Kann

Peter Wells-Grandmaster Secrets the Caro-Kann

Cyrus Lakdawala-The Caro-Kann move by move

Lars Schandorff-Grandmaster Repertoire 7 Caro-Kann

Start with Gallagher's book and then try one written by Lakdawala. Once you get some experience playing this opening with both colors you can read the rest.

Hopefully this helps. Leave a comment if you have a question or need further help.

Best regards and good luck.