A work like Capital may be a little much to start with and the style is a little old fashioned, but I think you’re underestimating yourself. Reading any sort of non-fiction just means taking it at the pace you are comfortable with. Sometimes you have to look up a word or a phrase, sometimes you have to sleep on something and come back to it with fresh eyes – that’s fine! Understanding isn’t something you get immediately – it comes over time. If you’re still intimidated, maybe try starting with something outside of direct theory. Pick a topic that you’re interested in, and find a book on it. Reading is like any habit, it gets better with practice!
If you’re worried about the length, maybe find a companion to it? Like perhaps, a history of their collection and publication?