For runs greater than 10K, it is important to maintain salt and hydration levels. Along with water, salt is greatly required to maintain your running performance. Regular salt can be taken once before the run and then during the midway of the course of the run. Unless your doctor has asked you to reduce your salt intake this is a safe practice. However, make sure you do a few practice runs with the salt before doing this during a major marathon. Some runners prefer to use dates with salt rather than only salt. Eating salty foods(nutritious balanced foods with a little extra salt) leading up to a marathon has also shown good results. Both can give a much-needed boost to your run.