From Born to Run, p. 92:
How do you make anyone actually want to run? How do you flip the switch that changes us all back into the Natural Born Runners we once were? Not just in history, but in our own lifetimes. Remember? Back when you were a kid and you had to be yelled at to slow down? Every game you played, you played at top speed, sprinting like crazy… Half the fun of doing anything was doing it at record pace, making it probably the last time in your life you’d ever be hassled for going too fast. That was the real secret of the Tarahumara: they’d never forgotten what it felt like to love running.
In other words, “If it feels like work, you’re working too hard.”