St. Simons will be an overnight trip. I doubt you can make it from St. Augustine to Fernandina in daylight this time of year. Measure the distance on your chart and multiply times the average speed you can maintain.
Also note that it's a bit of a distance from the St. Simons sea buoy to anywhere you might want to
anchor. The bottom is usually mud/sand (Danforth territory) with strong currents. For a night or two we usually deploy just our
Delta if we're happy with the location.
Unless all you want to do is sail, I'd recommend taking a relatively leisurely two days to Cumberland Island, stopping near
Jacksonville on the way. This allows you to
anchor up/down in daylight. If you make good time, you could
dinghy ashore and explore Cumberland for a couple of hours. It's gorgeous.
Have fun!
By the way, catching the tides at the inlets (if possible) will make all the difference on this trip.