I think that you're asking for too much improvement too fast. In the beginning rapid improvement comes quickly and easily as we gain fitness. That doesn't necessarily hold true as we progress. Every incremental increase in speed (endurance and power more accurately) takes a lot of work.

Since we're in the Training section bear in mind Sturgeon's Law. Yet I am certain that the 10% remaining is essential and that some sort of structured, focused training plan can lead to more certain gains. Probably not as quickly as you've seen so far though.