Training to Century Ride Day ?Mileage Span?

[cycliste]

I did my first centuries last summer, the Seattle to Portland (STP) ride. (Back to back 100 mile days.) My longest prior ride that year was around 40 miles; longest ever ride before that was 55 miles in 2006.
I hadn't followed a training plan, and hadn't had much time to prepare. I made it through the ride, I think, because of dumb stubborness, and because my kids were waiting at the finish line. I took my time, and ate small snacks frequently. I'll always remember the pepsi on ice I drank in a saloon somewhere along the road - it was wonderful. By the way, I discovered that I was able to accomplish alot more than my mind thought I could do, as long I could ignore the suffering.
I was sore and tired for a couple of weeks after. This year I'm actually following a training plan.
I'm 40+ years old, so didn't have youth on my side.