In my experience you have to build up to high intensity exercise as it takes a while for your body to adapt to burning fat for it. It's very easy to bonk if you do high intensity stuff with not a lot of glycogen in your system. I found (slow) running fine with it but with cycling I had to be careful as it was easy to push your HR and move from fat burning to glycogen burning.
@Drokk Have you had any issues with sleep? I found it hard to drop off without having some carbs in the evening.