When most people look at Ragen Chastain, they don't see a ballroom dance champion, a certified health coach, or a marathoner who's training for her first IRONMAN—a long-distance triathlon consisting of a 2.4-mile swim, a 112-mile bicycle ride, and a 26.2-mile marathon. They see a fat woman.

At 300 pounds, Chastain's appearance doesn't match the typical idea we have of someone so fit, but that's because "the typical image is bullsh*t," says Chastain, who posts about her training on her blog, Dances With Fat.

The idea that you have to be skinny and toned to be strong and athletic is fueled by fitness DVDs and magazines that never feature plus-size models, by athletic wear lines that don't go above a size large, and by fitness instructors who tell you to "do one more for the fat people at home," (something Chastain has actually heard in classes she's taken).

Despite the fat-shaming workout culture, Chastain and others like her have built careers in fitness as dance instructors, yoga teachers, and even pole dance trainers. And they've learned a few things after spending hours at the gym. Here's what they'd like you to know. (Want to pick up some healthier habits? Sign up to get healthy living tips delivered straight to your inbox!)