making chief - 8 to 16 years or so depending on how much you can "dig"
as for the math, brush up on exponents, sci notation, and logs. aint too much precalc i don't think, but I went back in 89. if you get really into it, you could read a book about heat transfer and fluid flow focusing on heat exchangers, pumps, steam generators; thermodynamics type stuff. but don't learn too much so you can learn it the Navy way. they want the answer they want on the test. math is good because it is pretty homogeneous across organizations. don't learn equations until you get there. just learn some general stuff that is ALWAYS going to be the same.
http://www.cheresources.com/heat_transfer_basics.shtml - pretty good.
http://www.mathsisfun.com/algebra/exponent-laws.html - good basics of exponents.
good stuff to learn is to KNOW COLD all the basics, like how to convert F to C, or what is a volt and how does it relate to other units of electricity. basics basics basics, don't get into any detail at all! don't go learning what entropy is, yet. I still don't know what it is.