Good job man. I don't know what is causing your sore back, but if you just started hills watch it on the down hills. Make sure you lean slightly forward and your foot strike is just forward of center mass. Don't lean back and over stride, which is what most people do. You may feel it more when you lift, but it might be the down hills.
The back soreness came after a session of power cleans last week. Like anyone, I really need to focus on technique. I'm feeling better today and plan to attack my workout... will post later.