With a legendary resume that includes everything from rock climbing to whitewater rafting to marathon mountain biking, Rebecca Rusch isn't just an ultra endurance icon—she has also been dubbed adventure sports' "Queen of Pain." (At 38, she won the first 24-hour bike race she entered, and has since gone on to complete the Ho Chi Minh Trail and part of the Iditarod Trail on a bike.) And yet she's always doubted her abilities and felt like she has more to learn—and cites that sense of self-doubt as the source of her success. Here, she talks the power in pessimism, why good enough is better than perfect, and the lessons pain has taught her.