I'm sure this is user-error and their MUST be an easy way to do this!
On all other Garmins I own (fenix, Edge etc), when navigating a route, there's an option to navigate back to the start - being pedantic this would be to the start of the activity recording, not necessarily the start of the route, although in most cases would be the same. On selecting you are given the choice to "track back" (ie. same route) or "direct" (ie. navigates you the most direct route), both useful depending on circumstances.
Yet I can't find this on the 66i?