... 20 years to worry about it though.
Someone on this forum told me a while back: let's eat this elephant one bite at a time. Go kick some ass in school and get on a boat. Go on some deployments and surge deployments, and maybe an extended shipyard availability (joy and pain, all). Give it some time and see if this Navy game works out for you. We ALL wanted to do 20 when we were in DEP. Some don't after some sea time. The NNPP can offer you a whole lot as far as working experience and wacky adventures around the planet, but it isn't for everybody. Make sure you set reasonable, measurable, attainable goals (short and long term) for your time in the Navy, and you might find that your true calling doesn't require 20 years service. A lot of folks here on nukeworker did six-, eight- or ten-and-out and have good jobs on the outside.
If you want to go enlisted and wind up teaching at a major university I'd probably recommend you enlist with an officer package on the horizon (you know, give yourself some blue shirt sea time and then shoot for commissioning). Officers don't have much (any) free time on sea duty (subs - I don't know about carriers yet), but I think they can have some awesome educational opportunities while on active duty if they perform well. They (the crusty old LDOs, NECP and STA-21 fellers on this forum) can tell you more about that.
I can only recommend that you don't enlist with goals set at Navy retirement followed by a college professor tour in CIVLANT. Take it piece by piece and make sure you can get what you want out of the Navy while giving the Navy what it needs from you. No good staying in if it isn't a mutually beneficial relationship. Most of all, have fun with it.