I'm starting med school in the fall, so I can't comment firsthand, but at my interviews I noticed that med students (esp. MSI) are among the most exercise-conscious people I've ever met. I heard someone else say that the best shape of her life she had during med school. Almost all my tours included the gym, or at least mentioned it. I don't think you will have to worry about it. I think time could be an issue during MSIII, but depending on the rotation, "walking the wards" could add up to quite a bit of activity.