It depends exactly what you mean by “intense”.
Overly intense exercise can lead to injury and other health consequences, whether you are fit or unfit. An unfit person will be more prone to that sort of thing, but it comes from overexertion, not just intensity. If an unfit but otherwise healthy person was doing a low impact exercise properly and had sufficient nutrition and hydration, intensity should not be a problem. It might not be the best way to exercise, as you would fatigue and not be able to exercise very long, but it would not be harmful in and of itself.
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