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?
All vintage, all the time.
Falcon Black Diamond
Gitane Tour de France
Kuwahara Sierra Grande MTB
Bianchi Super Grizzly MTB