Whenever I've let exercise slip to the back burner, when I start back up, I like to add in several sessions of hard Cardio to get my heart pumping again.  This week, I've been doing 50 minutes at a time in the gym: 20 minutes on the treadmill at level 4 (a brisk walk), then 30 minutes on the stair climber at level 60-70.  I make sure to bring my ipod or a magazine to keep me company for that