Just do cardio. I agree that you can get away with less or no cardio on a keto diet (once you're in ketosis), but it still burns calories so i keep it in the program. I can't see that it matters if those cals come from dietary or stored fat as long as you create a calorie deficit and remain in ketosis. So my advice would be to do your cardio anytime it fits your schedule. I still prefer am cardio because that's when I have the most energy.