I've recently found a passion for running, and had such a hard time training until I found Lake Merced. Lake Merced is an amazing place to run in San Francisco. The paved loop around the lake is 4.5 miles, with gentle up and down hills. The trail is fairly wide so you can run side by side with a running partner and there's still room for bikers or other runners to pass you on the side. There is also a small dirt trail that runs along the pavement for runners that like a little softer surface.

I've also gone biking here, and a couple times around the loop makes for a great ride. The scenery is gorgeous. The only downside is that there is a busy road that runs along part of the loop, so there can be quite a bit of traffic going by at times. But the primarily flat trail in SF and being outside totally makes up for it!

*Tips:
There are parts of the trail that are pretty dark from the shade of the trees when the sun is going down, so I would recommend going early if you can, or wear reflective gear. It can be hard to see other runners or bikers when it starts getting dark now at 5:00!