In 2012, Vanessa Garrison co-founded GirlTrek, an organization with a simple goal: get women walking for 30 minutes a day. Now 110,000 walkers strong, GirlTrek is a national force. The story of GirlTrek is about health, justice, power, and survival. But mostly it’s the story of trying to change your community, and the world, through something as simple as going for a walk. To understand how GirlTrek was started, how it blew up, and where it’s going next, Outside contributing editor Florence Williams takes a rambling walk with Garrison around Washington, D.C.