Steve,
I'm going to have to disagree somewhat with the posters above. If you want to do a metric century, you can. You could walk out your door right now and complete it. It's a matter of mind of matter.
This past October I got my first road bike. Before getting it, I had been riding my mountain bike some on easy trails, but never more than about 10 miles at a time and approximately 1/2 of that would be on pavement. I put a cyclocomputer on that bike at the beginning of June. As of today, it has 421 miles on it.
Now, as I said, early October I got the road bike. I rode a 5 mile ride on it. Then another 5 mile trip. Then a 10 mile one. Those occurred in the first week that I owned it. 1.5 weeks later I completed a moderately hilly metric. Miles 35-53 were the hardest for several reasons: first and foremost, that's when the hills got bad. But, almost as bad was the mental tiring I experienced. I had never ridden that far at once in my life and by mile 40, I was too far from the beginning to have that excitement and too far from the finish to see the light at the end of the tunnel.
However, about mile 54 or so, I could begin to see the finish line and actually finished up pretty strong. AND, most importantly, I was able to get out of bed the next morning by myself.
You have said you don't want to necessarily finish fast, just that you want to finish. If that is true, my advice would be to just get as much saddle time as possible.