You can see the total mileage of your route right below the name of your route. In case you completed the route, however, there are two things that you can do to view its total mileage:
Duplicate the route. A duplicated route should display total mileage right under its name.
If you are on Android, you can use the 'Print route' feature.
Circuit doesn't track you while driving, so there's no overview of the actual mileage. If you want to see your actual mileage, you can use Google timeline for that.