In conditions clear of snow, when I am staying mostly on trail, I rarely hike less than 15 miles a day. As my gear load gets lighter, I get bigger and bigger mileage.
I was doing 20+ miles a day consistently while through-hiking the Appalachian Trail 6 years ago with a pack almost twice as heavy as the one I now carry. If I could achieve that level of hiking conditioning again, I have no doubt my pack weight would allow me 25+ mile days consistently in the same terrain.
So unless you're getting into snowpack, and unless the trail is in horrible condition, 12 miles a day seems quite reasonable (even with pretty significant elevation gain). Remember, the folks at Backpacker are many miles from embracing lightweight backpacking, and they steer their readers toward conservative estimates for a casual weekend backpacker.