mkroyer -- hugs!!
As much as I'd like to stay always in peak form, at optimal weight, with life under control -- it just doesn't seem to work like that. I find that I go through peaks and valleys in everything. You're just in a valley right now. Quitting smoking is major, training for marathon is major -- it's a lot to ask of yourself!!
One thing that helps me through it is to try to manage my expectations. You are so talented (and goal oriented), and I think you tend to expect too much of yourself? I try not to set time goals for races anymore, just go out and do what I can - that day. Maybe I'll walk the last 2 miles, but I'll finish, and if that's the best I can do, then that's good enough. That approach really helps me to manage my anxiety. Enjoy the moment. There is no failure!
I'm 3 weeks post marathon now, and am ramping runs back up again. I was really tired out the first week or so, but feel pretty much back to normal now. My 13 miler on Sunday went really well, I felt strong and held a decent pace (for me). I'm also heavy right now (5 lbs over redline!!), which really affects my speed. Our weather is finally cooling off a bit ... we have a 16 miler planned for Saturday as now we start to ramp up for Disney in January.