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.
By the way, catching the tides at the inlets (if possible) will make all the difference on this trip.