I live and breathe mathematics. However, I never liked math classes much until the end of college (repetitive, pendantic, and slow-moving lectures) and usually ended up truant for my math-heavy classes. However, I learned some math on my own (ironically, after getting side-tracked while studying for the MCAT) and have settled on this area of research. If you're hoping to delve into advanced material and don't have the time to take classes, I'd highly recommend the Dover series (comprehensive review of each area of pure and applied mathematics). I've learned so much from those books since finishing undergrad...
That being said, very few of my classmates share my love of math. Of those who do, most are MD/PhD students or our class's superstars. If you are not a math-lover, don't worry--the only math necessary to do well in medical school is algebra (even then, it's debatible--I've started quizzing my doctors before they prescribe me any medications these days). And some of us mathy students love tutoring our classmates 🙂