Oh...I'm sooo glad someone brought this up! I'm the same...stuck at 25km/h (15mph) or so for ages. I've been riding seriously since 1999 and just seem to be stuck at this level. My endurance is WAY up...but speed and strength remain constant. When I was a competitive swimmer many years ago, it was the same. Speed and strength leveled out, but my endurance kept improving.
Maybe that's just the way we are? Thoughts, anyone?