I have found major success with fasted cardio. That is to say, timing when the exercise occurs relative to your most recent meal seems to dramatically modulate the effects.
If I do an hour of cardio within 12 hours of eating, it feels "normal". Nothing to write home about. If I do the same routine but I've been fasting for longer than 12 hours, I can feel my mitochondria light on fire after about 40 minutes in. The first few times I did this I stopped because I thought I was going to die. It felt super weird to me. Like a very strong parasympathetic response. You do get used to it though. I think this is one thing that can actually affect certain fixed points in your metabolism. If you go from zero to 80% VO2 max and keep it there for an hour on an empty stomach, that energy has to come from somewhere. And it has to happen pretty damn quickly. I think the time pressure for energy delivery under adverse conditions is what makes this so impactful.