Is it possible? Yes.
Is it likely to happen if you're a regular citizen? No.
The two cenarios would be: an attacker trying targeting you as a person, so the attacker would need to be able to exploit the cardio machine AND be able to exploit the iPhone. An attacker would only do that if they have very specific reasons to target you, since it's not a "profitable" attack. (selling the iPhone bug to someone else would be more profitable).
The other scenario would be something like stuxnet, where it's a virus that auto-spread across devices. The reason this is unlikely is because the virus would need to be able to exploit not only the iphone, but also every single type of cardio machines (or media centers). Stuxnet was possible because the systems were similar, but in this case the way to exploit and iOS device is totally different than exploiting a cardio machine or a car entertainment system.