Hi Ksrunr, thanks for confirming that white salt on skin is a bad thing. I saw that on myself after I noticed it on a lot of people around me, especially this 7 foot tall muscular guy. I also felt chills, which was weird too.
In addition to drinking water and Gatorade on the course until my tummy sloshed, I had a Camelbak Flashflo (48 oz) and I drained that too! Had no idea I would need so much liquid.
Next time I do a full marathon, it'll be in the autumn and someplace colder (not Chicago!).