My first century was in 1997 on my fully loaded Perth-Adelaide trip across the Nullarbor Plain. It was from Kimba -- the geoegraphical east-centre of the Australian continent -- to Port Augusta. It came after I spent a couple of days replacing the rear wheel after the original hub collapsed, taking out the derailleur and numerous spokes in the process.
It was a perfect day with a tailwind and reasonably flat country -- a few rollers to help boost the average
Go
here if you want to read about it.
My second century wasn't for quite sometime afterwards, about three years, I think. It was a "reconnaissance" ride for another ride I was organising for a social bike group. I rode from home, fully loaded in anticipation of probably spending overnight in the bush, did the recce over gravel roads, then road home. It was about 161km -- I just couldn't be bothered stopping to put up my tent.
My third, in January 2002, was a practice run over 200km from the north of the Tasmanian island to the south to introduce me to the concept of randonneuring with the ultimate aim of PBP-2003.