If you don't mind the drive through the tunnel, check out the Lafayette Reservoir. It's open sunrise to sunset, patrolled, and very safe. And, of course, GORGEOUS. The inner run (2.7 mi) has some moderate hills and the outer run (4.5?) has some bigger ones.

If you BART, you could get off at the Lafayette station, run to the entrance, up the hill (*that* hill's a killer), around and back. It's about a mile to/from the station, or a little more. That gives you 5+ or 7+ miles.

The fire trail in Strawberry Canyon is always good. Top part is flat, bottom part not so much. But if you cross Grizzly Peak at the top of Centennial you can park in a lot in Tilden. The trail isn't a loop but there's a few 'logical' places to turn around making for a flat 4, ~5.5 miles with some hills in the middle, or a 7.4 mile total if you run the whole thing. If you start at the top, it's mostly flat for a little under 2 miles out, then 2 coming back. Turn around when it starts going down and you're in for a flat 4 miles with a great view. If you continue down on a gradual downhill (there's also a steep connector part futher down which marks the turnaround for the 5.5mile loop) , you're gonna get a good climb coming back up, but it's not too bad and you're rewarded with the flat two miles coming back.

Parking there isn't as good as it used to be. UC seems to ticket up at the Space Sciences lab now. 15 years ago they didn't. There's also some parking at the base of the trail, below the botanical gardens but above UC's pool. It's hit or miss if there's a space there and you're going to have to start running up hill to begin with, but for the long loop, it's more rewarding to get the downhill at the end when you need it most.