I think once you are where the OP is you have to challange yourself.

You can: 1)Increase cadence 2)Increase weight.

They use the same principals with Spinervals and a lot of the people who race have had very nice increases from Spinervals.

Then again I'm no PRO

I'll say this...I was under the impression I was doing OK with my riding 20-21mph avg for 15 minutes....once I got my computer with cadence it was a different story...it changed my riding style a lot.