Did you do your recent 70 miles alone?
I find that riding with a group - such as on an organized century - is MUCH easier than solo rides. More to look at, there are breaks at rest stops, and folks to talk to. The miles fly by.
I don't think it's too much of a jump to go from a 70 miler to 100. Sure, it will be a challenge (isn't that the point?). But if you did 70 in snotty weather, I think 100 in (predicted) ideal conditions will be a snap. You seem to have a good solid base. Just pace yourself and use those rest stops.