Great job on your first century!! I'm not going to attempt one this year, but am training for more endurance.

The only way I know to get stronger is to do intervals. I do intervals where I put on as much gear as I can and still maintain a cadence of 75-85 and keep my heart rate below lactate threshold. I work in this tempo state for 10 minutes, recover for 10 minutes... I do 3 or 4 of these a few times a week. I also do an interval at a little bit higher heart rate, but for shorter duration -- 6-8 minutes each. Always recover in between.

I would love to do squats with weights to build strength, but my lower back is pretty fragile and although I might wreck it on the bike, I'm doing something I love. I would be really p-o'd if I wrecked my back doing squats and couldn't ride.