It can be done, but there are a few things you have to do...
1. Skip the start of the 70mi.
2. Be at the start line for the 45.
3. Get on your bike and go!
The folks who run the tour told me once that the routes were pretty much open all day and that you can mix and match or customize your ride as you please.
I think route sign-ups are mostley to get a good idea of how many folks will be on the course at the time and for stocking rest stops.
I had to make a route change at the Lilac Century in Spokane a few weeks ago,I signed up to do the 25, but the nice lady with the mic said,"you don't have to wait! The 50mi is rolling out in just a few minutes!" UGH! My quads are starting to hurt again....