I agree that the ride is more mental than not. You do have to have a base, of course, but I've known people who have ridden 100 without training much at all. I'm not saying they felt all that well when they finished, but they did it.

I train more for comfort than anything. I'd rather finish the ride and be in control instead of struggling to finish also, I love to enjoy post-ride beer and food.

Take it slow, pace yourself, and ride with friends. Oh, and partake of rest stop food and beverages! Be sure to take in enough carbs and fluid.