Let me preface my comments with the following disclaimer... I haven't done a century yet. I just got my 1st road bike 2 months ago and I'm working on building up distance (most of my rides are typically in the 15-25 mile range).

I think your idea of intervals is a good one. I always read about intervals doing great things for increasing your aerobic capacity. That will make you stronger for your longer rides.

I also agree with Grog's suggestion to take occasional breaks on increasing mileage (i.e. not EVERY week). I've read that is good too.

Best of luck! I'd like to be century capable, so I might check out the link I saw makbike post.