Opinions differ but in my view, if you are drinking water and eating food you are doing the right thing. There is nothing in the drinks that you won't get from other sources. I might recommend electrolyte replacement if you were doing > 4 hours in high temperatures, but not otherwise. It's mostly hype.

Try them if you like, but my guess is that your first thought was correct. You run out of steam because you're tired, and are still building endurance.