You would be surprised how your Dad may react!

A lot of military people are pro MJ and/or used to be smokers...

If your Dad is a high ranking enlisted man, than he has seen what alcohol and other drugs can do to people, because of his experience with his own troops.

You're not a kid anymore, you're a man and you're making a decision that you feel is best for you; I'm sure Dad will come around eventually, even if he doesn't support you right away.