Like plenty of people who enter races I am motivated not by winning or finishing but by beating my own PBs in each distance. To date I've been pretty successful in this personal duel with my former self but I'm beginning to sense that some of my PBs are about to become unbeatable - in other words, I'm not getting any younger. Anyway I was wondering if not PBs what other system of motivation people use. Is there some kind of age ratio I can use so as I continue to better myself but on some kind of sliding scale of decrepitude? If not time then what other targets do people aim for?