I've never cycled the entire route but I and others have ridden on the roads they use at the same time as the event.
Be prepared for people cheering you on from their front doors as if you were a competitor. I found it amusing as I rode past on a tourer with 2 panniers hanging off the back
If you’re going to do it whilst the event is in progress with participants riding round you just ask yourself whether it’s fair/sensible for you to be there if the road is actually closed to traffic and be willing to accept any disapproval that may come with it.
In my experience, the roads aren't closed to the general public, just motor vehicles.
5-6 of us cycling along never met any disapproval of any kind and more often that not were greeted by both competitors and race marshals.