movement_violation

I am currently an undergraduate Mathematics student who is interested in Linguistics, having taken six classes since I started Linguistics late in my undergraduate career. I have a full-time job next year outside of Linguistics, but I want to continue with my Linguistics studies in my free time, with the possibility of one day getting a Ph.D. How do I continue with Linguistics in the future in a productive way? I have a number of books that I would like to read post graduation, but would it be better to enroll in a night course (I'll be in the MD/DC area) and possibly generate a writing sample? Or, should I be doing something different from both of these ideas? Thanks for the help!