I rode about a dozen or so centuries every year for a couple of years until a few years ago. It also felt like a challenge and I was usually concerned about an upcoming century.

Then I entered my first 24 hour ride five years ago. The goal was just to do longer than I'd ever done (108 miles). Managed 252!

Since then I've become a randonneur and did the super randonneur series in 2010 & 2011 (200km, 300km, 400km, 600km). And the first 1000 kms of PBP (sadly dnf'd at second to last control). "Only those who risk going too far can possibly find out how far one can go."

For me, breaking thru the barrier of a century was the hardest part. Most "long" rides that we see and hear about are a maximum of 100 miles. I thought that was the limit. Once I learned that I could ride beyond 100, the long rides have been much easier. Now riding a 100 miles is just a ride like any other. (I rarely even think "I'm riding xx miles." I think "I'm riding 42 miles to the next control.")