How intense is the math aspect in API/primary?
It ain't much. When you learn Aerodynamics and Engines in API, you learn some equations (lift, Bernoulli, venturi, etc.), but you really only need to know how different variables are related within the equation. You never really do any solving of math problems that you're probably used to. Math in Primary is mostly cockpit math, which isn't very hard because you have to do it while you're flying. Either way, you don't need to be a math whiz to make it though API and Primary.
Also, how did you guys warm up for flying?
You'll hear the term "chair flying" a lot in primary. Just set up a profile of what you want to during your flight, and simulate it. Go from engine startup to engine shutdown. What switches are you moving? What are you saying on the radio? What are you looking for on your instrument display? What should the plane be doing if I do x/y/z? Always try to anticipate what's going to happen next so you're ready for it. If something happens that you weren't expecting, it can really throw you off, so always try to anticipate, but remain flexible to changes.