Burn-Out and Marathon Training

I have felt this way for a couple weeks, but today solidified the fact that I am burned out with running.  I just feel done.  I love running so I don't want to give it up completely, but I am done with training.  I had a horrible half marathon this morning that I probably shouldn't have done because of a knee injury (I think it is just overuse), but I did it anyway and probably made the knee worse.  We will see what the doctor says about it.  I was just using this half as part of my long training run for this week so I went into it forcing myself to go slow, but because of the knee, I ended up a little slower than planned---and I didn't feel near as good cardio-wise as I thought I would.  Oh yeah.......amid all this burn-out and being ready to quit, I am supposed to run my first marathon in 6 weeks.  I ran a 20 miler three weeks ago and felt really great.  I was supposed to do 22 today, but barely eaked out the 13.1.  I am just tired of it.  I am TIRED of long runs, I am tired of injuries, I am tired of everything.  I am seriously ready to be done today.  I feel disappointed that I have worked for so long (I have been training since November) to run this marathon, and now I don't really care if I just give up.  I guess I am almost hoping the doctor will tell me I can't do it--at this point I wouldn't really care, while a month ago I would have cried my eyes out.  What to do, runner friends, what to do......Any words of wisdom, advice, encouragement, or support to quit?  I will probably feel different in a few days, but I just can't see myself doing the marathon anymore.  How can I do 26.2 when I don't even feel like doing 5--ESPECIALLY with a knee that is very unhappy?  All week long I have been debating whether or not to jump in on a marathon that is in 2 weeks because I felt ready now, but my run today told me that was not an option.  I am hoping that if I decide to go forward with training (and the doctor allows it), I will be ready in 6 weeks.  What do you do when you just want to quit?