You may not think you have enough time to exercise. But you can work activity into your daily life—you just need to be committed. Take 10 minutes out of your lunch hour to take a walk. Walk to a nearby diner or pharmacy or library instead of driving. Make it a habit to take the stairs instead of the elevator. Park in a faraway parking spot instead of the closest. You'll be surprised at how fast these little changes can make a difference.
Some people really can't walk very far and tire out quickly with exercise. Instead of becoming discouraged, resolve to do what you can do and work to make that a regular habit.