Crusader Kings 2: Capturing invader's lands?

If you are the defender then you can't get any land from winning the war, assuming he used claim CB. If you win, you gain some prestige and gold, while the loser loses the claim, some prestige and gold.

I don't think It might be worth prolonging the war if you are not a Nord, the "Levies are raised to long" stacks fast and take a long time to fade away, even after the war. If your vassals start rebelling and you are in the middle of a war, things can get bad really fast.

Probably too late, but I think it's important to note that defenders do not suffer the "Vassal levies raised too long" relation penalty. As such, I would actually advise that you keep up the war for the time being.

Why? Because you should actually be getting a bonus to opinion with your vassals for "Defending against foreigners" or something like that. This could be extremely useful for when your 82 year old character dies, as the heir will inherit the war along with the relations bonus. This relations boost can be huge for mitigating the "Short Reign" penalty until it falls to manageable levels.

Edit: I suppose I should probably answer the overall question as well. No, there is no way to take land as the defender in CKII. However, as I pointed out, there are other ways to take advantage of being attacked.

As the defender in the war you cannot gain land. He will lose Prestige (and possibly pay you gold depending on the CB) for losing the war. You should disband your levies and accept his surrender.

However, if you have any valid CB to press against him, you should do so immediately after the war ends. Truces are unidirectional, he may not attack you, but you are under no obligation to return the favor. Once the war ends his garrisons will begin reinforcing, but they will be quite some time recovering. For the next year or three his armies will be small, so winning another war against him will be trivial unless he gets outside help.