We all know we need to get off our couches and on to our feet for a good sweat, but despite the fact that working out makes us look great, feel great and live longer, 50 percent of people who start an exercise program drop out within six months. "Exercise is hard work and people want quick fixes," says James Gavin, Ph.D., a sports psychologist and professor at Concordia University in Montreal, Quebec. "But a quick-fix attitude doesn't lead to long-term commitment." Here's why—and what you can do about it: 1. You narrowly define exercise. Most of us have a…