I'm going to take the opposite opinion of some others and tell you not to worry about it until it comes up. Go to the interview as planned, and don't bring it up at all. I would not expect them to surprise you with a urine test at the interview, but I wouldn't entirely rule it out either. I would more likely expect that they require it within the first week of work, or that you schedule one with a testing facility.
If at some point they tell you that they will be giving you a drug test, then be honest about it. Try something like this:
Actually, since medicinal marijuana is legal in my state, I have a medical marijuana card to help me treat some personal health issues. I know that it can be detectable up to XX days after consumption, so there is a slight chance that I would fail that portion of the test. I understand that marijuana is not legal in this state though, and I most certainly would commit to not using in the future. If there would be problems with my test coming back positive for marijuana right now, could we delay the test until a later date?
You want to be as factual and informed as possible. Know how may days it is possible to be detected. Know the laws in both states. Explain your illness if you feel comfortable. Show that you are a responsible legal user, not just some "pothead".