The main source of fuel for low to moderate intensity exercise is adipose tissue (fat). Once your heart rate climbs to 80% or above, the body has a more difficult time metabolizing bodyfat for energy, thus it switches to glycogen for instant energy. So to burn glycogen for fuel, you need to get your heart pumping, and sustain it at that level for a period of time. Any cardio or high intensity resistance training will do the job.
Remember, once you're in ketosis, there's no reason to perform high intensity work more than a few times a week, and certainly no longer than 1 hour. Your performance will suffer and you risk losing lean muscle.