I'm an old army Soldier but i have an opinion on everything, especially the navy haha
Pros and cons to staying 20 years. For one, you can expect retirement income starting at 20 years when you retire. that means if you retire from the navy at age 40++, you will draw retirement pay until death. You can set it up for your spouse to draw it if anything happens to you. Depending on your rank enlisted or officer, the return on investment is substantial. This will seem simplistic, but it's all notional anyway. To be a pharmicist, you will need a long education which translates to a commission. I don't see them keeping you enlisted with a pharm D. If you retire at 20 years at the rank of Commander, I could see you drawing $4000+ a month in retirement pay. How much money do you need in the bank, 401K, Roth IRA to get you the same income? I don't know, ask your financial advisor (far worse scum than a recruiter) or your bartender or accountant. I would guess you are pushing a million bones to equal pay and health care benefits for life. So in a sense, a military retirement is like retiring a millionaire. But you don't have principle to dip into. (Sorry kids, nothing to leave you when i pass so go work like everyone else.)
There are many in congress who feel this entitlement is wrong, since other professions don't get to retire immediately with pay after 20 years. (they are not talking about themselves). Not going to say firefighters and police officers don't deserve their pay, THEY DO, but here's the kicker. Military duty is DANGEROUS! You are floating on, flying in, and driving equipment made by the lowest bidder. Oh not to mention 90% of the world hates you, including (pathetically) far too many americans.
Go see a Navy ROTC recruiter to discuss how to get in as a pharmicist. he will probably tell you to have a 3.99999 GPA, be physically overfit, have at least one of the Huntsmans as a friend, etc but it can be done.
Don't let anyone tell you that you cannot make your dreams come true. Go find a way to make it happen.