I realize this thread is old but I'll put a bit of extra information on boosting sodium levels or other things to help out with POTS. I have a few friends with it so I thought I may as well share a bit of what they learned so as to make someone's life a little easier.

The low-calorie way to get to get in a lot of salt is through via canned chicken/beef/vegetable broth or tomato juice, adding extra table salt to savory dishes (1 teaspoon is 2300 mg sodium) or via a low-sugar electrolyte replacement drink like Powerade Zero or G2. Nuun tablets that dissolve to make a fizzy drink have been really helpful as well as Saltstick capsules, as they contain a mix of electrolytes rather than just sodium. One Saltstick capsule is the electrolyte equivalent of half a Nuun tablet. Drinking more liquid is important too. Eating big meals or sugary foods can make things worse as can hot weather. Paradoxically, some people with POTS have hypoglycemic swings and should carry some dextrose tabs with them just in case.

As mentioned previously, the Mayo Clinic website has some great information, especially for adolescents.

The compression socks are meant to help keep prevent the pooling of blood in the lower extremities thus maintaining the amount of blood circulating to the heart and brain. I hope the OP got through her race alright.