I could tell the difference on a 10km run. The difference was around 10% extra time + a feeling of my legs being heavy. No "torture", though. It wasn't 100%

I could tell the difference on a 10km run. The difference was around 10% extra time + a feeling of my legs being heavy. No "torture", though. It wasn't 100% predictable -- sometimes I felt fine fasting (and ran fine).