10k

I'm running my first 10k this Saturday at the Los Alamitos Race on the Base. While training, I notice that my legs get tight around the 5 mile mark. Is that just my threshold I'm approaching or would salt or something help? I was thinking of mixing a little Gatorade in with my water. Or is a 10k too short to do anything other than drink water and train harder? Any advice would be appreciated. Thanks!