The idea behind training is simple. We stress our bodies beyond their current capabilities, and our bodies respond by healing the stress and adapting to become stronger in order to better deal with future repeats of that same stress.
What surprises me though, is how completely unpredictable that adaptation is. Sometimes our bodies adapt in a way we may not notice it. Sometimes our bodies just maintain status quo and don't really adapt at all. Sometimes, very rarely, they will react in a way that adapts beyond what is expected.
That's what happened to me today, in fact. I'm a very new runner, and as such my normal tempo training run pace is only around 10:15/mi. Today, when I ran that pace it felt like I was running much to slow for a fast run. My heart agreed, as it stayed on the very low side of zone 1. So very carefully I increased my pace until I was in the upper end of zone 1, knowing as the run progressed that would break into zone 2, which was the workout I had planned for today.
I finished today's workout with my HR much lower than normal, and my 3 mile time was 28:06. That's 9:22/mi. Not only did I run faster, but I gained efficiency, burning about 60 Kcal less then normal for this run.
This was of course a happy surprise, and I'm sure it will be one I look back on when I'm not as new, and the big increases in performance slow down.