For Samantha Livermore, exercise not only makes her feel better — it even makes her nicer. “I’m so much happier after a long, invigorating run,” says the 23-year old college student who interns at a granola company. “Probably more approachable, too!” A devoted runner since sixth-grade gym class, she now dashes through San Francisco areas like Golden Gate Park, Twin Peaks and north of the panhandle. Samantha is eager for the Nike Women’s Marathon, but admits the challenging landscape is a bit worrisome: “I grow more terrified of those hills by the day,” she says. But dressed in a racer-back tank, mid-calf pants and good socks, she’ll be ready. “I feel invincible,” she says of her racing philosophy. “Nothing can stop that attitude.”