I wouldn't say 'training' is necessary for centuries, I did 192km (just short of 120 miles) on a whim a few weeks ago... All I ever do usually is ride to work and back or bike to wherever I'm going (my bike is my transportation), no actual training of sorts... Just work up to it, I'm usually around 5:30/6 hours for a century (fixed or not I seem to ride the same speeds over distance right now), if you take it easier and make a full day of it you'll fare well.