Out in the baylands is flat and nice, if you don't mind the occassional landfill aroma Out in PA by the recycling center there is entrances to the baylands (the "Moonlight Run" has a nice 10k and 5k route, from what I remember, if you can find their map). In a few months, Bair Island in Redwood City (Whipple Ave and Hwy 101) will reopen; that's a ~5k loop. Very exposed, but very flat. I saw someone mention Sawyer Point by Crystal Springs reservoir (north of Hwy 92 and 280), and that's a real runner / walker haven. marked path every 0.5 mi. enjoy!