I think Darcy's advice--about doing a metric and then a century--is good.

Originally Posted by
Catrin
My calculator tells me that if I add 6 miles a week to my "long" ride then I will be up to 200k by the end of September - that seems a reasonable way to build my mileage base.
That makes it sound pretty doable, although if you end up being like me, the mileage progression isn't quite so linear. Like you, I accomplished a pretty significant goal within my first year of cycling in that I bought my first road bike in July of 2006 and did a 160-mile one-day ride exactly a year later. In building up to that mileage, I experienced certain mileage "plateaus."
During my first summer/fall of riding, I got up to about the 45-mile mark without too much fuss. After that, it took some consistent weeks of training to learn how to eat and drink and to get used to the aches and pains that set in after about 3 hours on the bike. After I got used to 45 or so miles, I quickly ramped up to about 60-65 miles (roughly a metric), again without too much fuss. From there, my body needed to adjust again. I spent some time "exploring" the 4-5 hour mark. How to eat, how to drink, how to stretch, when to take a break and when to grunt it out. The season ended at about that point, and I more or less put the bike up for the winter and worked on building my aerobic base with spinning.
When spring came, after a few somewhat painful weeks, I quickly ramped up my mileage back to the 60-mile mark. I then started to also do some speed work. For me, being in the saddle for long hours was a lot easier if I was riding with other people, but most of my riding companions were faster than me. If I was going to ride with my friends, I needed to get faster. So, mixed into my long weekend rides, I added some weekday training rides. My strength and endurance really improved at that point.
It was at this point that I did my first century over Memorial Day, 2007. In preparation for it, I was riding about 150 miles a week and my longest ride to date was 75 miles two weeks before. It was a relatively hilly century and that, probably more than the mileage, made it hard. I finished, but there were times during the ride that it really hurt. It was my first real taste of the mental toughness required for century plus rides.
It took a good week to recover, but after the century, I kicked it up a gear and started riding 200 or more miles a week. Most of my long rides were in the 65-mile range (because that's about the longest ride CIBA offers), but I usually rode that distance both Saturday and Sunday so my body was adjusting to long hours in the saddle. I was also finally able to do some hilly rides in Southern Indiana without feeling like I was going to die, too.
By the time my first year anniversary came around, I felt pretty strong and decided to do the RAIN ride on a bit of a whim. Some of my friends thought I was in over my head, but I felt confident enough to try. I'd also note that I was pretty nervous, too. It was all I could do to keep my breakfast down. 
As it turns out, I felt like a million bucks during that ride. A nice tail wind and relatively comfortable temps helped, but I just felt inherently strong. I did it with a friend who had just finished a cross-country bike trip, and she made for an excellent partner. We did the ride with a 19 mph average and finished in the top third of the finishers. I have to admit that it was one of my prouder moments. I went on that year to do my first week-long tour in September, riding about 400 miles in six days in the hills of Tennessee. Again, I was pretty proud of that.
So, I wholly believe that we can set our sights on pretty ambitious goals. That said, you really need to listen to what your body wants and needs. It likely won't be as simple as doing longer and longer rides each week. Your body will need time to adjust to certain hurdles. If you push too hard, it will push back. But what's the worst that can happen if you try? You train hard and decide ultimately to put it off until next year? So what? There's no shame in that. I will add this, too. It is a lot more fun to do a century (or more) for which you've adequately trained. Yes, it's possible to gut them out, but there's no real joy in it, at least for me.
Live with intention. Walk to the edge. Listen hard. Practice wellness. Play with abandon. Laugh. Choose with no regret. Continue to learn. Appreciate your friends. Do what you love. Live as if this is all there is.
--Mary Anne Radmacher