The ride last year was 83 miles, some years the route is closer to 90. I was very out of condition, the weather was not good (really humid, some rain, strong headwind) but it took me about 6 hours on the bike. I averaged 13.5 mph. This pace put me in the bottom half, but people were coming into Shiner as late as we stayed (3:30).

A long ride will definitely help you get ready, you don't have to do 80 miles but I would try to do something around 60. There are a lot of threads here about preparing for a long distance ride that are very helpful.

And don't let me scare you, I cannot emphasis how little riding and training I did before Shiner. You will have a great time on the GASP.