I assume you are writing about the women: I don't think that the majority of them (porn) do much at all - they are just kind of young. How much does a 18-25 year old have to do to stay "in shape"? LOL
There are a few out there that seem to really do aero/anarobic activity: Jill Kelly, Sydnee Steele and a few others.
I do think that the softcore models take better care of themselves: Nikki Nova, Sandee Westgate, all of the Playboy models.....