Which variety of Emergen-C are you using? There are about 20 different formulas. I haven't gone through their whole website, but what I've looked at in the stores really doesn't have an acceptable electrolyte balance. Most of their formulas tend to be lacking in calcium and sodium, in particular.

More to the point, I don't like the idea of using Emergen-C as an electrolyte replacer just because it's not a good idea to get so much vitamin C (especially in ascorbic acid form) during exercise!

Carry some calcium-magnesium capsules (not tablets, too hard to digest during exercise) or mix a soluble supplement like Endurolytes.

If you prefer real food, I've been doing well on my long runs with blackstrap molasses and salt added to my rice syrup.