Why blow your dough on an overpriced, yuppie-infested gym when the hills of San Francisco offer stamina-building opportunities free of charge? If time is of the essence you can even confine your workout to one city block, the steepest and most challenging of them all: Filbert's stretch of vertiginous real estate between Leavenworth and Hyde. (When David Letterman came to town several years ago, his most memorable stunt involved dumping a truckload of Ping-Pong balls down this cliff.) The entire eastern flank of Russian Hill from Bay to Pacific is daunting enough for any urban climber, but with its 31.5 degree grade, this block of Filbert is the neighborhood's own Matterhorn. The sidewalks are pleasant tree-draped staircases, with handrails along the trickier sections, and while you're panting your way uphill you can rest midpoint and enjoy the palm- and cypress-covered gardens at number 1141. At the hill's crest is a lovely view of Coit Tower and Treasure Island and, better yet, several Hyde Street watering holes just one block downhill.