I sort of did the same thing. This summer I never did any rides over 50-60 miles and I did a century in September, with no special training. The day I wanted to do a longer ride of 80 miles, I was sick. I had never ridden more than 70. But my rides include a lot of climbing and the century I did was fairly flat. The last ten miles were very hard, but it was more because of the 30 mph headwinds and a freakish microburst of very cold rain.