Breakthrough Tempo

Not sure where this came from but I just had quite a huge breakthrough (for me) on a tempo run. I always do these by heart rate, not allowing myself to get over 90% of my maximum heart rate reserve in order to make them 'true' threshold runs and not time trials. The last few I have done have all been in the 18:15 - 18:30 range (6:05 - 6:10 pace). Today, I ran 17:39 (5:53 pace) even though my overall beats-per-minute was actually 5 bpm lower than on my previous best 3 mile tempo. It took me a mile to get up to 85% and I was able to cruise between 85-90% for the rest of the workout without slowing down. Lately my tempos have been slight positive splits since I'm quite diligent about not letting my heart rate get over 90% even if that means slowing down, but today I was able to run very even, staying in my target zone. Splits were: 6:00, 5:49, 5:50. The last time I was running this well was about three years ago when I was sub 17:00 for 5K. I'm hoping this is a good sign. If I can ever run even half as smooth as NCAA cross country champion and 5K record holder Lawi Lalang (above), I'll be back to my sub 17 ways.