I think people with Parkinson's need to have a 'prescribed' exercise routine. Just like medicine that must be taken "on time every time", exercise is really important for general health and symptom management.
Dr. Jay Alberts has written many articles on the effects of forced exercise in managing Parkinson's symptoms. He uses stationary cycling in his studies.
I have found that a treadmill provides similar benefits for me. The speed of the treadmill is faster than I would normally walk, and it does not slow down as I have a tendency to do. So I am forced to walk beyond my normal ability. Thirty minutes at a time, every second day seems to help. Even better is to do it every day, but unfortunately, I find it too easy too neglect my 'forced exercise'. I get to be too busy with other things such as gardening, yard work, housework, or sitting in front of the computer typing my thoughts about exercise. These are all good things to be doing, but do not seem to help as much as the treadmill does.
So, I need a prescription for my exercise and a way to keep myself consistently doing it.