Heidi and I did it last year and we did the 75 mile route. We didn't make it to Hell's Gate in time (they shut it down 1 hour early!), so they routed us to the 75 mile route.
Just pace yourself and go your speed. Drink and eat and rest as needed. Don't let the group pick your ride for you. Be OK with not staying with them.
I have to say, overall the ride is not difficult. There are little to no hills... the only hard part is the heat, the chip seal, and the headwind on the way back into town. OH, and the scenery... it's dull as can be out there.
Well, good luck! Trust me, you can do it. Don't let it get into your head. It is a hard ride for people who aren't use to heat... but if you live in/around Texas, you will be fine.
"Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather, to skid in broadside thoroughly used-up, totally worn out, and loudly proclaiming: WOW WHAT A RIDE!!!!"