If one can do 70-80 miles, I agree 100 should be within reach. A big factor will be whether the route is flat or hilly. I did a solo century last month, on a mostly flat course with only one significant hill, at 15 mph when in the saddle. The last 20 miles at much reduced speed with my back wheel wobbling from two broken spokes. I'm not sure I could manage it on a hilly course. I had to pack my own nutrition, and there were a couple of stops where I could replenish my water, plus I had a backpack with a frozen bottle of Gatorade that stayed cool for a very long time, very refreshing! A supported century will be much easier as you won't have to carry anything. Some rides may also offer flat and minor repairs if something goes wrong.

Be careful though, if you do latch onto a group; 6 years ago I rode the "Ride to Conquer Cancer", two 90 milers in two days. On the first day I was riding in a group to near the back to get a little bit of a draft. I noticed that these were clearly not people used to riding in a peloton (and for that matter nor am I) so I backed off and within a few minutes of doing so there was a major crash with much of the rear of the peloton going down. Broken bones, ambulances, etc. I was far enough back by then that I wasn't involved, and I finished the course solo. Better to work a little harder but finish than end up in a crash.