OK, so I'm out of my league in this thread. But what I would be likely to do in your situation would be to build distance gradually, and include 1-2 hard tempo rides each week. Not sure what a tempo ride distance would be -- I've only used this strategy running. A tempo run, training for marathon distance, might be 6 miles, 8 tops. So a hard tempo ride? I would say between 10 and 20-25 miles -- so very short compared to the distance you're training for, but at a pace significantly faster than the rest of your training. I might run a tempo run :30-:45 seconds/mile faster than my easy training pace (I think technically it's supposed to be more, but I can't ever hold myself back that much on easy days

). I really can't translate this to pace changes in riding but I think you know it when you feel it. It's not as hard as traditional "sprint" type intervals -- it's a pace you can hold for the entire workout distance, but one that is pretty far above "conversational" level, and you should feel pretty spent at the end (like you wouldn't want to do another "interval"). The idea is that you gradually train your cardiovascular system to be able to work at higher and higher sustained intensity.
Just an idea... good luck!
Just reread your original post. To clarify, I was thinking specifically of increasing speed in your century and decreasing your recovery time. For some of the other things it sounds like shorter harder intervals might be better?
But again, I'm totally outclassed in this thread so...
