It seems strange to me that the SSGT would just leave him in limbo for five months without actually putting him in contact with his immediate superior, someone who I think would be really helpful in this situation. If it were me, I'd tell them that you hadn't decided on either the Navy or the Marines. You are of course going to get kind of a sales pitch, but if you're applying to be an Officer of Marines, they'll tell you everything up front. They'll help you get there if you want it, but they don't want to waste their time if you're not 100% about what it is you want to do. I'm not sure how the Navy works in that regard.
Also, if you're going to do this, you need to start PTing. Just the standard three mile run, pull-ups, and crunches. There's another wierd aspect of this whole thing, the SSGT would want you to start PTing right away if you're going to put an Officer App in. If I were you, I'd call him back and see if there's someone in your area that's in the same position as you are (applying). Then you two can PT together which is easier than doing it alone.
Main point though, I'd just be up front with them. If you're looking for a way to not piss people off, tell them the truth from the beginning. That way they can't take it personally if you don't decide to go through their pipeline and into the other one. Always be totally upfront, the people you're working with will be able to help.
To answer openbah's question, you do contract with the Marines before your Packet goes to the selection board, but if you're not selected, obviously you're released from that contract. That's just a paperwork anomoly, but it's a good thing to keep in mind if you're considering the Marines, you take the oath when you contract which is before your application is submitted.
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