I think this is it. It is indeed satisfying to see and feel progress. For me, that progress was being in less misery with each subsequent run (and losing weight). I can see how time improvement would give others that same satisfaction. Thanks!
For the record, I hate running - at any distance, any speed - but I do a 1.5 mile Cooper Test 3 or 4 times a year just to get an honest assessment about my cardiorespiratory fitness level, and adjust my other exercises accordingly. Much to my dismay, there is no other exercise modality that quite stresses me the same way as plain running.
