
Yep, ditto for me! I think it's extra hard with Disney races, too; pretty much every training plan assumes - and encourages you - that adrenaline and excitement will make you faster on race day than in training. And that may well be true... but Disney races are crowded. And have things to stop and take pictures of. And sharp turns that require slowing down. I'm always much slower in a Disney race than in good training runs, but have turned in a PR in every non-Disney race I've run. It's for that reason (well, and because I can't afford runD anymore, lol!) that I'm planning to do more non-Disney long races after GSC; I want to really see what I'm capable of!
It mostly depends on how motivated I am to mix Nuun in the middle of a run.

My summer long runs usually end up like this:
Pre-run: down 16-24oz Nuun
First few miles: drink two 10oz bottles of Nuun (mixed at home)
Next few miles: drink two 10oz bottles of water w/ a little leftover Nuun (because I have a swish of Nuun left in each and don't want to lose time to pull out a fresh Nuun tab, so I just add water at a fountain)
Last few miles - realize I've stopped sweating. Pull Nuun tablet (already cut in half) out of waist belt and attempt to remove sticky cling wrap - drop one half of Nuun tablet on ground. Pick up, blow off dirt and decide to use it anyway. Open top of bottle #1 with teeth, toss in 1/2 a Nuun, add water, close cap, put bottle back on waist belt. Open cap of bottle #2, drop on ground. Pick back up, rinse off in fountain, add 1/2 Nuun, add water, replace cap, jam onto belt.
The intent is to drink Nuun the whole time, but it seldom actually happens.