Actually smoking MJ probably isn't the issue. The issue is that you (and others) actually tried to circumvent the laws and intentionally focused your life (at that point) on acquiring a substance that doesn't stand well with LEO's.
To many departments, it can look as if you focused your attention and wasted your life back then with getting a MJ "medical" card, instead of focusing on planning your future and career. I think departments would think, "what else would you weasel yourself into?"
I'm not picking on you because I'm sure by this post you've started that you're a completely different person. However, many LE departments or HR people don't really care about somebody that's "changed for the better", when there are plenty of candidates that have done everything "right" in their lives to become a LEO.
In todays terrible economy where "everyone" that's young (and old) is begging for a good career type job, LE departments are back in a position of advantage. The departments can carefully pick & choose a "better" candidate than one that maybe has a "slim shady" past. These days, the economy and LE departments are not struggling for "good" candidates like they did pre 2008. I'm being blunt and straight forward with you by playing devil's advocate here.
As others have said already, don't try to hide it and don't try to sugar coat it during a background check or interview. Also, don't dwell on the subject either. Discuss it if it comes up by stating you made a mistake that you're not proud of. You learned from it and have changed your lifestyle completely. ...next question.
After all that, you can only pray for the best outcome of your quest.