The thrill and satisfaction of improvement (setting a PR) can provide positive reinforcement, feedback and motivation.
If you really benefit from competing and pushing yourself, and enjoy setting PRs, try this to set new PRs every season as you get older.
Every season (whatever a season is for you, but its best if it is 3 months long or longer) repeatedly run over a new different course of your choosing for time. Select a course you like (long, short, flat, hilly, trails, roads, etc.). Do not measure the course and turn do not use a GPS to measure your pace.
It should be a course you do not know the distance. You will only be competing against yourself and the course for time during that season, and how far you run in a certain time is irrelevant.
Tempo your first efforts over this course, and train to improve your times over this course as your season progresses. As the season progresses, try to gradually run your course a little faster each time. Depending on how long your course is, you might try to run over it for time once every one to three weeks.
As you learn, train for and adapt to your course, you should cover it faster and faster, setting a new PRs for that course as your season progresses.
Its good to note and remember a few splits at landmarks (not at measured distances) along the way. This will help you guage and compare your efforts as you go. If you repeatedly run your course many times, you will want to experiment with different strategies (going out very hard, even effort, finishing hard, or ?) to see which work best for you.
For each new season, select a new different course and you can set PRs forever.
No need to ever measure how far these courses are. You really do not want to know how much slower you are running as you age. Just enjoy the satisfaction you get by trying your best and improving just a little bit on your course every season.