You will be able to do it as long as you keep riding regularly, as AZfiddle says. I was in your position last year. I think my longest ride prior to the 65 mile ride was around 49. The end was tough for me, but at that point, I thought well why would I quit with another 10 miles to go. I also rode around 14mph avg. (If that!)
Will you be riding with anyone? that certainly helped me. I know on the other thread you mentioned you don't do the group rides often, but they may help you hook up with someone around your same ability. Or maybe even more advanced then you, but willing to hang with you on the ride.