The San Francisco Bike Map and Walking Guide which indicates hill grades on city streets by color, is a great resource. Online, check the San Francisco Road Runners Club site (www.sfrrc.org) for some recommended routes and links to several local running clubs.
The city is spectacular for running -- Runner's World magazine named San Francisco number one in its list of the top 25 running cities in 2005. There are more than 7 mi of paved trails in and around Golden Gate Park; circling Stow Lake and then crossing the bridge and running up the path to the top of Strawberry Hill is a total of 2 1/2 mi. An enormously popular route is the 2-mi raised bike path that runs from Lincoln Way along the ocean, at the southern border of Golden Gate Park, to Sloat Boulevard, which is the northern border of the San Francisco Zoo. (Stick to the park's interior when it's windy, as ocean gusts can kick up sand.) From Sloat Boulevard, you can pick up the Lake Merced bike path, which loops around the lake and the golf course, to extend your run another 5 mi. The paved path along the Marina runs 1 1/2 mi (round-trip) along a flat, well-paved surface and has great bay views. You can extend your run by jogging the paths through the restored wetlands of Crissy Field (just past the yacht harbor), then up the hill to the Golden Gate Bridge: its walkway is 1.7 mi long.
First run in 1912, the 12K Bay to Breakers race (415/359-2800. www.baytobreakers.com), held the third Sunday in May, is one of the world's oldest footraces -- but in true San Francisco fashion, there's nothing typical about it. About a third of the 50,000 to 100,000 runners are serious athletes; the rest are "fun runners" who wear famously wacky costumes -- or no costumes at all (there's a faithful nude contingency). The race makes its way from the Embarcadero at the bay to the Pacific Ocean, passing through Golden Gate Park. Bring your camera; you won't believe some of the getups!
The San Francisco Marathon (415/284-9653. www.runsfm.com), held annually on a Sunday in late July or early August, starts and finishes at the Embarcadero. Up to 7,000 runners pass through downtown, the Marina, the Presidio, and Golden Gate Park, and cross the Golden Gate Bridge, tackling some of the city's milder hills along the way. Related events include a four-person marathon relay, a 5K walk-run, and a half-marathon.