In the old days I used to regularly stay out late, drinking vast quantities of various forms of adult beverages. Somehow I always, or almost always, made it to work the next morning despite how I felt: tired, worn out, nauseous, shaky, achy, dehydrated, dizzy, confused, etc. The usual morning after payment, often day after day. By the time I changed my indulgences from alcohol to marathons I was no longer used to staying up late or drinking much or having hangovers.
Imagine my surprise after my first few 20+ mile runs when I woke up feeling exactly as as if I'd been partying all night. Vaguely flu-ish, somewhat ill, all those symptoms without the indulgences. It made no difference whether I actually had any alcohol or not. I'd still feel hungover. But unlike a regular hangover I didn't feel better the next day. The dizziness and confusion would be gone but the aches would be much worse. I'm sure there's some scientific and medical explanation that goes beyond the stresses imposed on the body by running for hours and the damage to the immune system and dehydration and squinting into the sun and wind in your face and ... wait, maybe there isn't any other explanation.
I'm not going to stop, despite feeling like crap afterward. When all goes right then it doesn't matter how I feel the next day, or the day after, or even the day after that. I'm convinced that once again running will get easier, will feel good, will give me that certain, unexplainable feeling I get from running a marathon and nothing else. I have to believe.