Marx was working with the idea of LTV, but it preceded him. It was the idea that labor is what transforms the inputs to the outputs, creating increased value in the process of production. If you follow this logic for all production, you end up with the conclusion that all of trade is just exchanging labor-modified things and services, and even just labor at its fundamental level.

Capital is Marx's critique of political economy. It takes this idea but uses his own formulation embedded in his own definitions of the commodity and value, taking great pains to be clear that he's not taking a simplistic view that, say, the value of a bicycle is not simply how much accumulated labor it took to build it. People who are bad at building bikes would produce a more expensive bike, but that doesn't change the value of bikes in general. That depends on aggregates and averages, including how much labor it takes to build a bike on average, how many bikes are needed/wanted, etc etc.

Importantly, Marx frames value as a calculation deriving from a supply-side viewpoint. It is the owner class who are obsessed with value and, in particular, how they might use it to maximize their profits in order to survive competition. To do so, they (1) own the means of production and (2) oversee the workers. They are monitoring and adapting the value creation process (workers turning inputs into outputs with expected higher commodity market sale prices) to decrease the cost of inputs, including labor, and they understand it to simultaneously be an expenditure that can be decreased by simply paying workers less money while also knowing their whole operation is dependent on their work.

This leads us to Marx's theory of exploitation, eventually, but at this point he's more or less done with LTV. He used it to point out labor's part in creating value, that it does literally everything to transform inputs into higher-value outputs, and did so specifically to point out how this functions under the capitalist system where labor is not in control of production.