For me, joining this forum was what got me started. Barely anyone on here talks about 100kms, so I didn't consider them. The first ride I did more than 40 miles was around JJs (now Terry's) End of Hibernation route, guided by google maps on my phone. The first event I entered was the Mildenhall 300.
I think there are plenty of regular commuters out there who become sportive riders. For instance, on a non-bike forum I go on, a bunch of people do the Wiggle series, the Fred Whitton and the Dynamo. Mostly, they know nothing about audax, despite it being well within their capacity.