I've done C25K, Bridge to 10k, successfully been running 5k @26 mins, 10k @54/55 minutes. Today I could barely scrape together 4k, it felt like my legs had been taken off and put back on wrong!
I sometimes (often) find the first 1k hard, but then get into my rhythm and I am away, but it didn't happen today I was literally counting down....
I slept well, I had no wine, had an early standard breakfast (small banana and half a piece of toast), why would it feel SO HARD?