Dr. Fung writes that exercise is fine during extended fasting.
I think it's fine, and it may will help you lose weight quicker; but it might not help you strengthen your heart or arm or leg muscles.
If you want to build heart or lung or arm or leg muscle, you might need to eat protein after your exercise session is over.
If you eat keto (20 or fewer grams of carbs per day) after an exercise session, it might be much easier and less painful to resume fasting again after the meal.
I do intense bike rides towards the end of fasts. After an hour or two of bike riding, I break my fast with maybe 100 grams or so of sunflower seeds. After some more bike riding, I might go home and eat a high-protein meal. Then I might resume fasting again.
From what I've read of Dr. Fung's writings, cardio only has a minor effect on weight loss, because it increases hunger. I'm not sure about weight lifting. I dunno if it has a larger effect or not.
As always, if you ever feel weak or bad during a fast, try consuming some electrolytes, then waiting an hour. If that doesn't help, I strongly recommend breaking your fast. My modified safer-fasting guide tells you one good way to consume electrolytes, if you're willing to do a modified fast.
[Edit: Dear all: Is there anything you dislike about this comment?]
I’m not someone who exercises regularly. I’m going to start doing that this week by attending group classes that are free at my university. But this other class is required and I’m paying for it and it’s worth credit. I can hardly exercise regularly so doing such a high intensity work out while fasted just doesn’t seem realistic to me. Even if it’s something I planned on doing.
And like I said, keto is a realistic LIFE change for me. I want to eat well for the rest of my life. Just well balanced and below my maintenance.