I have been struggling to run the times in workouts the last couple weeks that I want to almost every time I try to run a workout. So I went back and looked at what I did last year leading up to my 5k and 10k PRs. Wow! I am certainly in better shape than I was back then. I had a few good workouts but not the volume of 5:0X and 5:1Y pace that I have this season. I simply set my expectations rather high, and while I am close to nailing the work required, not reaching those expectations worried me. However, comparing the work done to the previous work done shows that progress is being made.
Performance is relative to expectations and experience. Those two are often at odds with each other. Performance relative to expectations is often lower than desired while performance relative to experience is [hopefully] higher than it has been in the past. The same goes for engineering. A project I expect to take 15 hours might take 30 hours, but two years ago it would have taken 60 hours.