I think I found what I've been looking for!

2 liters cold filtered water
1/2 cup organic lemon juice
6 tbs fruit juice sweetener (such as Wax Orchards Fruit Sweet, but agave nectar works well too)
1/2 tsp. sea salt


No refined sugar, no artificial flavors or colors . . . I'm going to try this out for tomorrow's 50 miler. I'll let you know how it is!