In my personal experience, it is impossible to do high intensity exercises in a long fast. I “bonk” really early into the exercise. On the other hand, I have much more energy for low intensity exercise - walking, leisure bike riding, muscle training with my own weight or light weights. Let me just reiterate: this is my personal experience, I have heard of many who can heavy lift during fasts.
There’s an explanation to what I feel: during fasts, we rely on fat rather than glycogen for fuel. There’s very few glycogen reserves in our body, but they are very easy to be used fast, which helps with intense short exercises. During a fast, we have to use fat, which takes longer to be transformed into energy, but there’s A LOT of it - 1 kg of fat is almost 8000kcal, enough for a good training day for Michael Phelps at his prime. So you don’t get to pull a lot of it at once, but there’s a steady flow from a very large deposit.
It might help if you become fat adapted first which can take 6-8 weeks or so I’ve heard. Go full keto on your refeeds and do only light to moderate intensity exercise until you become fat adapted. There are some athletes that swear by it. You could try it and see if it works for you.