My first century about 14 or 15 years ago, was about 10mph average for the day. And a long day it was. Began at 7am and finished about 6pm - lots of stops to rest. Pulled back into the parking lot as the trucks were loading up the port-a-potties. We were last in on the ride, but there were quite a few that had finished just ahead of me.
And now, after becoming a much stronger rider, my fastest century so far has been about 7 hours but I don't remember my average speed. I've never been fast and probably won't ever be, but I don't let that stop me from doing those long organized rides. They are fun, no matter your speed. At a 10mph average, it would take you 10 hours. If you build up your "distance" so you are able to ride that many miles, you'll be able to do it, just at your own speed.
Just pick a century ride that you want to do, register for it so you are committed to riding it, and start increasing your distance a little each ride. There are lots of good training guides out there - I used this one while training to ride a two-day STP: http://shop.cascade.org/content/even...gested-mileage - and if you'll notice, it mentions mileage, NOT how fast you ride those miles :-)
Have fun :-)
Edna




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