Well, he's more likely to know than I, how far you can push without crashing your metabolism. And you're more likely to know than he is, assuming you're paying attention to your own body. I only say "gentle" because it's better to stick to that when unsupervised. Personal trainered exercise is different. You have someone paying attention who has a pretty damn good idea of how far to push things, and who will tell you (or bloody well should be) if you're doing something wrong.