The ride is in 2 weeks away. Oh wait... it's next weekend (21st).
I'd say fix wheel the first day, and freewheel the second day. The hills around Bastrop the second day will warrant a freewheel. You can muscle up them... but the downhill and transfer to the following hill will be MUCH nicer if your speed isn't limited so much.
OR, seems like the 2nd half of day 2 gets pretty flat.. been a couple years since I rode it.
That's assuming you don't mind working on the bike a bit, and your chain will work for both gears (if you decide to use different size cogs).