I’ve heard this from almost every runner I know, yet I have never ever found it harder to exercise because of a hangover - in fact, I’ve done many of my fastest 5km times after a heavy night. It curtails neither my desire or ability to get out and walk/hike/run — on the contrary, I often feel like going for a run will be a surefire way to cure a hangover (and atone for the behaviour that caused it).