I don't think you need any "help" with your parents, after all, it is your decision, and probably one of the first/biggest you will make independent of them as an adult. There are many on the board, who simply announced their enlistment contract and ship date as facts. Family legend has it my own DH made the announcement on Christmas Eve just as they were heading out the door to Mass (as in, oh, by the way...)
I think it is great that you are getting involved early and choose to attend DEP functions and begin your PT regime. I don't think you can ever be in "too good" shape for boot camp.
As far as stereotypes...there are only Marines and anything else is irrelevant. You will be judged on how you perform and your character and integrity. Nothing else matters. There are Marines from all walks of life and from all backgrounds.
Marines are my kind of people. They have a plan to kill everyone they meet.