Of all the supposedly “essential” running equipment I use, there are two tools that I couldn’t do without: Sporttracks software and my Garmin GPS watch. Using the two in combination, I’m not only able to see my progress from day to day, but also from month to month, season to season, and — perhaps most dramatically — year to year.
Take a look at this chart. It shows three pieces of data: Average pace, average heart rate (% of max) and average length of run.
Notice how the average pace and the average heart rate have dropped as the average distance has increased? Funny that.