Last year for Darkside I ran the 5k & 10k at my normal pace but stopped a ton for pictures, characeters, mile markers 10k 501st, random stuff. Stopping for pictures provided enough rest breaks that by running my normal pace I wasn't dead at the end or ruined for the half. Then for the half I wasn't off the pace I wanted until the last couple miles, the humidity in April got to me a little more than the previous 2 races. I'm always a little stiff at first on multiple race weekends, but once I get going I'm fine Like, everyone else said training back to back days helps get you ready, I run 6 days (sometimes only 5) a week so my legs know tired. Now fatigue as in falling alseep earlier at night, yes definitly. Last year I'm pretty sure the night before the half I fell asleep like at 6:30/7:00 easily, we always get up early enough to get the earliest bus so that fatigue was there causing early bedtimes, but that was good so I good a full nights sleep.