Croft again - Speeding it up

Im up to around 500 miles of base training so far this year and so i feel the need to start pushing a bit harder. A motivation for me to push harder and ride faster is a saying i heard . . .  'Train slow and you ride slow'. An example could be a guy that rides his bike to work everyday at the same 14mph average never stressing his systems will never be a fast rider although he rides everyday. 

Garmin 705 Virtual Partner

I have a usual training route i use around Croft, Warrington of about 23 miles. The route and statistics have been saved as a course in my Garmin Edge 705 bike computer/sat nav. I can call this course up or any other that i have ridden such as the 'Old Lags Rivington ride' and race against a virtual partner that rides at the stored rides previous pace.
Yesterday, Sunday i went out for my usual training route around croft and loaded the course i had saved with an average speed of 16.1mph. So i knew to beat the virtual partner i had to go faster than that for the 23 miles. Most of that previous rides heart rate was in the low end of zone 3 (144-164bpm) and averaged 147 bpm and so it was hard work this time out with my heart rate spending an hour in zone 4 (164-185bpm) and averaging 165bpm. As a result i could fly along at 25mph until lactic acid built up which made me slow down again to about 16-17mph then after a minute or so i was off again. I guess it resembled an interval training ride rather than a constant pace higher 'Tempo' ride. I beat the virtual partner (mini me) by almost 3 miles and knocked 10 minutes off the previous rides time. It was a good training ride and i feel that i worked hard and achieved something since my quads are still aching today.
Stats  were -     miles 23.34, time 1hr 17s, Average 18mph, max 32mph, Hr av/max 165/177 bpm.