Your comments are interesting regarding folks with advanced PD being able to easily ride a bicycle. What fun! I have read that people with balance issues are often able to ride a bicycle and that in fact bicycle riding can benefit folks with balance issues (can't remember the study or where I read this - but have a pd-friend with balance problems that swears by bicycle riding).
On a related, but different dimension, there have been studies by the Cleveland Clinic that indicate that riding a tandem bicycle with a non-PD participant who 'forces' you to ride harder/faster than you thought you could has had very positive results regarding a slowness in progression. It's referred to as 'forced exercise'.
So ... keep on pedaling!
Life is a dance. Don't sit it out. ---- Jackson H. Brown