There are some weird results in exercise science related to this. For example, if you perform exercises only on one side, unilateral strength on the other side improves (something that seems like a pointless novelty until you consider training while injured). There's a huge amount of strength that relates to 'neural' factors.
Conversely, apparently elevated heart rate due to gaming does not improve aerobic fitness.
So I'm thinking that, broadly, there will be no such thing as a free lunch here (as per usual).
I highly doubt it. At least not more of a workout than you get from just bouncing in one place, which is more than a lot of people get.
I wonder if anyone has tried using this tech to somehow trick the body into activating what happens when one actually goes running? Can I get a workout by just watching myself run in this VR world???