That's tough. This question is all but impossible to answer.

I could ride 50 miles very hard without food and not feel too awful bad at the end. I could never do the same thing for 100 miles.

I think if you're doing 100 miles for the first time you should throw "very hard" out the window. You want to know you can actually go that far before you start pushing for speed. I do at least a century every month and next week will be the first one where I'm actually concentrating on speed. You have to build up your endurance first.

Even if you're trained pretty well you'll always be able to average a higher speed for 50 miles than for 100 miles, even if conditions are perfectly equal, which will never be the case. You can go anaerobic in a 50-miler and it won't catch up with you later as much as it would for the 100-miler.