Exhaustion

My log is suddenly filled with, "Tired & sore." "Very tired." "Very, very tired."
I've recently had sharp pain in my right foot, but then they go away, come back, go away - it's been a managable issue. I change my stride and form and I keep going. Then yesterday the same pain started in the left foot, but it refuses to be managed.
I was so extremely beat after yesterday's 18M+ run. My muscles are sore as if I've run a race. I almost felt ill. I slept deeply and felt like a train wreck this morning.
I've had to take more time stretching. My muscles are so tight, when I sit on the floor with my legs straight in front of me, I can initially only get my palms to my kneecaps! After considerable relaxation and reaching, I finally get my palms wrapped around my toes. Even though I hold this stretch for a minute, my muscles tighten back up again as soon as I'm done.
This is overtraining. The timing is okay, I guess. I have a 50K coming up on Saturday, so I'll be taking it easier than usual this week.
This body is on the verge of disintegration.
Maybe after a few days of slacking, I can get that good feeling back.