Hi Beth,
Here's a funny story for you. When I was about to turn 30 (back in the Stone Age), my brother and I signed up for a century. I trained for it -- boy did I train for it! I devoured books by the editors of Bicycling Magazine, Greg Lemond, you name it, and came up with a training plan for the century, which I stuck to. My brother, on the other hand, did his own thing -- played hockey, went out for the occasional bike ride, and even did a super long ride less than a week before the century.
The day of the century came, and about 10 miles in, we hit some hills. My brother went up them no problem. I had a few issues. (Jobob, if you're reading this, you've probably guess that this is when the infamous Fig Newtons made their presence known on the Calaveras Wall.) My brother continued to ride along thru flats, rolling hills, etc., no problem, as I struggled to keep up.
I must admit that I was a little pissed.
I kept my mouth shut and kept going. Really, it was a lovely ride -- we got to know some great cycling routes in the area and met some very nice people at the rest stops. Anyway, about 10 miles from the end of the ride, I felt great -- plenty of stamina to finish the ride, and go a little further if necessary. My brother, on the other hand, was whupped! He was definitely looking forward to the finish line -- once he got there, he was getting off of the bike!
So, what's my point? Since your goal is to ride a century, and it sounds like this is your first century, I think you're doing the right thing by training for stamina and distance rather than speed. It's a really, really good sign that you haven't injured yourself and are still looking forward to your weekend rides. It sounds like you have a good fitness base, so if you want to incorporate some speed workouts, go for it! Push the envelope for a minute at a time, and rest for about 5 minutes before doing it again. But if they take away from your enjoyment, you should probably continue to focus on building your distance.
I should also mention that one of my goals this year was to improve my speed. I found a really nice set of workouts that I was doing on my indoor trainer during the week, and I noticed the improvement during my outdoor rides on the weekend. Unfortunately, this put a little extra strain on my lower back. This combined with being a weekend warrior at softball practice has resulted in me blowing out my back. Guess what? I've been off the bike for the past few weeks, and when I do get back on the bike, I'm going to be as slow as ever. Oh well, it's not a race out there.