Speedy folks *will* go by you - and some of them will have the courtesy to call out "on your left!" as they do. (If they're close enough you'll get a nice break from the wind as they stream by.)
Other folks will be going at gentler paces, though - lots of people do them on hybrids and sometimes on really eccentric vehicles.
A hundred miles **is** a whole lot longer than a metric century, though. It's more than 150% of it - it's not like converting from yards to meters :-)
A rule of thumb I've heard is that on a supported, find-your-pace-and-cruise, rest-stopped ride, where you pay attention to drinking and eating and heat and cold and all that, you can go about 3 times as far as you can comfortably go on a "regular" ride. So, not three times as long as the longest ride of the week, where you're pushing yourself - but three times as long as a sort of reasonably fun challenge. Your own personal mileage is going to vary a *lot* though - get to know where your bonking point is and what's most likely to trigger it.