I have seen two different doctors for my P.D. Both of them stressed the importance of consistent exercise---walking, cycling, whatever you can comfortably do. This goes back 11 years when I was diagnosed and my first neuro suggested trying to do some type of exercise daily. I find this to be true in that whenever I'm sick or unable to walk for exercise each day, I feel a definite weakness in my legs when I resume the daily 30 to 60 minutes of walking. It's not scientific but it works for me. I think some studies have been done, using the wii games to generate daily exercising (playing tennis & various sports). You may not notice a difference at first, but if you stick with it, I think you'll see some benefit.
Good luck!
Mari