Here's the GPS route and map for this ride. If you come at 7am, the route is a little under 53 miles, if you come at 8am, the route will be 38 miles.
If the weather doesn't hold out, there's a cutoff to reduce the mileage by about 10 miles. Right now though, it looks like the rain won't start before 11am, so a 13mph pace would get you back in time.