I did a 5K recovery run. Slow, nice and easy. I am feeling much better. Last night I had some good sleep. The recovery run went well, took around some 45 minutes. I am coming back. I think my body reacted to the initial shock of the 20k (which was characterized by depletion of all my glycogen stores and energy stores and my heart pumping thick, heavy blood - because I was dehydrated) by trying to shut me down lest I kill myself. I think that is what the post-marathon depression is all about. Its the body trying to slow you down. Self-preservation can be very primal. So it tries to slow you down and keeps you out of sorts and kills the feel-good hormones so that you don't drive it through such a difficult task again.
But I am much better now. I can resume my workouts as normal. I am now considering doing a 10K this Sunday to set a PR (Personal Record). Maybe a Tempo run on Friday.
I think the dark cloud is clearing fast. Phew!