I would second some of the advice about working on a set of small bugs or features. At my current job we have 2 million+ lines of code in a proprietary language

I would second some of the advice about working on a set of small bugs or features. At my current job we have 2 million+ lines of code in a proprietary language with absolutely no documentation! I found while working on smaller issues it gave me an opportunity to familiarize myself with the rest of the codebase. I also drew a diagram of the class hierarchy as I went but quickly ran out of paper. It's a daunting task but you will become familiar with the code in no time. Good luck!