Daily goals of exercise are hard for me because if I miss a day, I feel like a failure, and that's a bad start to the next day. If you're having problems going 20 minutes every day, try 40 minutes 3 or 4 days a week. Figure out which days work best, and which times work best, and treat the exercise like an appointment with yourself -- an appointment you have to keep -- until you build a habit. Soon, your body will crave the exercise.