No, not dumb, provided that you pace yourself well. Start easy and stay easy. Start well hydrated and stay well-hydrated. Start with a good breakfast, and stay fueled up thoughout the day. Don't set yourself a time goal, but instead, set yourself an EFFORT goal. You want to keep it easy and steady all day. Keep the gearing comfortable, keep your breaks to a minimum (unless you are overheating, in which case a break in the shade with your feet in a creek is a lovely thing :-) ) Expect your average pace on your century to be less than your average pace on your metric. And that's OK!

Increasing from a metric to a full century is a 50% increase in distance, so pacing is important. But you have a solid base, so it's definitely doable.

In randonneuring, we ride a spring series that consists of 200K, 300K, 400K and 600K. Most of us who do this year round, do a ride of a minimum of a 200K (125 miles) every month. Some of us do that 2-3 times a month. But over the winter, we rarely ride over 200K in one go. But come the spring series, we ride a 200K, then a 300K a few weeks later, then a 400K a few weeks after that, then the 600K a few weeks later. But we don't ride "interim" distances to prepare us from the step up from 200 to 300, or 400 to 600. For example, I don't ride 250 to prepare for the 300. Rather, I use all of the pacing and fueling strategies listed above to keep me at a low steady effort for an increasingly long time.

In my case, this is what my riding has looked like since January. Note that I do continue to ride between each increase in event distance.

KM Date Elapsed time
130 1/5/2013 8:15
114 1/13/2013 5:58
207 1/22/2013 9:36
112 1/25/2013 5:37
202 2/2/2013 10:35
100 2/3/2013 6:05
200 2/17/2013 11:18
200 2/24/2013 11:27
208 3/3/2013 11:28
200 3/9/2013 10:58
200 3/23/2013 10:16
114 3/31/2013 5:18
300 4/6/2013 16:07
213 4/13/2013 12:23
200 4/14/2013 11:30
300 4/20/2013 13:29
400 4/27/2013 21:20
365 5/3/2013 24:00
211 5/18/2013 12:40
202 5/19/2013 13:27
600 6/1/2013 38:26
204 6/15/2013 11:53
600 6/22/2013 34:50
201 7/6/2013 11:50

My point here is that the body is generally capable of much more than we think it is, provided we don't over-exert. I've read in SO many places that the "safe" or "typical" way to increase mileage is by no more than 10% per week. I'd have to call BS on that though. As long as you pace yourself and fuel yourself, and you are healthy, you'll be surprised what you are capable of!