Hey Kim! If your cortisol has been tanked and you're on the upswing from that (yay), I would DEFINITELY recommend going the walking uphill on a treadmill route. It's not so much about "burning calories," it's more about what happens inside your body as a result of the exercise you do. Walking operates mostly off fat oxidation, so is extremely effective for body fat loss. And it's not so intense that it puts a strain on your adrenals. HIIT is also great for fat burning overall, but it will run mostly off glycolysis - meaning it will run mostly off stored carbohydrate in your muscles and liver. The only thing is that if you are already pushing the line on overstressing your body, HIIT can intensify that, and too much stress on the body will take you the opposite direction of where you want to go. So I would MOSTLY do the uphill walking, and if you want to try doing one day a week of HIIT, see how you recover (did you sleep worse that night? Feel worse the next day/have lower energy?) and if you aren't recovering well, hold off on HIIT, or at least the intensity of your HIIT, til you're a little further on your healing journey. If you recover well from it, keep doing it once a week and walk the other times you do cardio. Hope that helps.