Most of the time, hydration is not necessary during training runs. On the other hand, I believe it is important for marathon training to practice what you will drink on race day. For example, at the California International Marathon (CIM), Precision Fuel & Hydration (PF&H) provides on-course hydration. You could practice drinking this exact drink, the exact amount, at the exact intervals (drink stations) as the race to simulate race conditions and avoid cramping, etc.
You may also wish to carry water on long runs on hot summer days. Dousing your head in the heat is nice too.
Maybe try the Naked Running Band.