Eat at the Double Horn across the street, then hit Bluebonnet for pie to go. You won't be sorry. The Double Horn is, by itself, a good enough reason to take 281. The Mac 'n Cheese is about 900 cals per serving from the cream and butter and cheese, the CFS and CFC are great, and the burgers are Yuge and delicious. Shrimp and grits are good, too, if you like it hot.
You'll be full as a tick, but get the Bluebonnet pie to go anyway; it's worth the trip across the street! Banana Cream, if they're not out of it...
Sounds like an overnight stay in Marble Falls is necessary for this trip. Arrive in time for lunch, then dinner, then breakfast. And my personal Bluebonnet favorite is the lemon meringue pie. You should, though, always have what you prefer.