My very first century was in high school. During the summer vacations I visited a friend who lived south of Munich, riding there by myself on my French Motobecane 10 speed touring bike. I lived some 130 km north of Munich at the time. I navigated with a paper map and followed the river Isar through the city center. When I finally arrived at his place in the southern suburbs I had clocked up my first 160 km distance. This was after doing Sunday rides of up to about 100 km.
After the weekend I rode home again. About 30 km from home I ran out of energy. So I stopped to get some food and drinks and rested in the shade for a while. To my surprise I recovered so well, I decided to make a detour and visited a cycling friend of mine who lived 25 km northwest of my parents. I then rode home from there. The total for the day came to 220 km. I didn't do another century for over 30 years.
After I got my Bike Friday in October 2011, I started working my way up from 50 km. In February I did a ride of 155 km, just short of a century, but that was after a crash... The first century after more than 30 years then was my first brevet training ride with a friend, which came to 227 km (training for a 300 km brevet). Like on my first 220 km ride as a teenager I found that when you run out of gas, simply eat and drink something and you will be able to keep going :-)