The principle of progression states that in order to increase fitness you need to apply a level of overload to the body within a certain time period. Put simply, you need to increase your workout levels enough to get results but not so much that you get hurt.
You are probably familiar with progressing frequency and intensity of your workouts. Progressing frequency adds workouts. Progressing intensity adds difficulty. This is like adding weight to the bar or reducing time to completion.
Whenever you beat your previous best you achieve a personal record or PR. It doesn't take long for a fitness initiate to become enamored with chasing PRs. This is partly because they happen seemingly daily. Should that person continue exercising for years they will run into the law of diminishing returns and PRs are few and far between. But this is usually only applicable to PRs based on the progression of things like weight on the bar or time to completion. I coached and exercised this way for many years until I found a better way: to focus on the progression of time.
I got burnt out and hurt chasing the wrong PRs. Once I realized that every time I enter the gym and finish a workout I just set a personal record in the 'Mike's number of workouts done' category I began enjoying exercise again.
Rather than worry about constantly improving arbitrary numbers I focused on improving one: my time spend.
By choosing to do something physical for one hour per day I rack up 365 hours annually. Every time I finish another workout that tally goes up by one.
It feels good setting a PR every day. To those who fail to notice progress I suggest this approach.