I'm in a similar position. I will be starting my PhD soon and there will, at some stage, be some programming. Although i did programming modules in my undergrad i have never used it for anything and that was years ago. I suggest that you are completely honest about it with your supervisor, there may also be a course in the grad school you can do.
I told my supervisor about my situation when we first met to discuss the PhD. He was really nice about it- he said we can learn together. I think we may also be using Python. In my undergrad we were using Fortran.