Maybe it's the heat. The temperature in my area has been over 100 degrees for several days -- 108 yesterday. Do you ever just get burned out with your training program? I don't know if I'm physically burned out...perhaps it's all in my mind.
To be honest, I haven't been cross training. Not really, anyway. I walk my dog daily and I occasionally stretch. I don't think it's enough. Instead of running 4 miles today, I am going to take a yoga or strength training class. Perhaps if I mix up my routine a bit I won't feel so mentally exhausted.
Mental discipline, in my opinion, is one of the most important factors of success in marathon training. I've gotta keep my brain in shape too. hehe