If you are re-starting your fitness routine after weeks or months or longer, simplicity will keep things manageable and enjoyable. Keep your workouts shorter to begin with (15-20 minutes) and add 5 minutes per week to what you are doing until you reach about an hour). After 3-4 weeks, you can add small amounts of intensity (10 x :30) to keep things interesting. This works for walking, running, swimming, biking or any other endurance activity