Waking up, your glucose and insulin levels are very low, that's a good thing because if they are high you won't burn fat effectively. And as they are no longer available, your body uses your fat to produce energy for your physical activity.
30 minutes cardio is all you need a day. They are worth it
How intense?
When using a treadmill at a 5 or 10 percent elevation, at medium intensity for 30 minutes is a great routine, remember to have breakfast shortly after you're done.
Benefits
Fasting cardio won't only help you burn fat easily but will improve your metabolic rate, becoming more efficent at transforming your body fat into energy.
Boosts your metabolism, so you'll keep burning fat after you excercise.
Eliminates that hard-to-get-rid-off fat on men and women, in areas suchs as: hips, abdomen, shoulders and lower back.
Can be done by almost everyone, it doesn't matter your physical condition.
Keeps you active throughout the day and produces substances like dopamine, serotonine and endorphins feeling relaxed and in great mood.
Walking is also an option when you don't have a bicycle. Other nice choices are jumping rope, and remember you don't have to be an expert the idea is to keep your heart rate up.
Tips
Do it as soon as you wake up, if you take long it might not be as efective.
Use an open and aired space, you need enough oxygen to keep up.
Use some loose clothes to feel comfortable and stretch out.