A short, finished list rather than an endless feed: 100 of the best fitness quotes ever written, picked out of the full fitness archive on QuoteForever. You will find Ralph Waldo Emerson, Jack Lalanne, Jillian Michaels and Johann Wolfgang Von Goethe among them.
Read the list top to bottom, or use it as a starting point - every author name links to everything else they said, and every quote has its own page you can share or turn into an image.