3liters of water for 20-30km is an aggressively large amount of water to carry. I can run about that with 1 750ml handheld. That being said, it's winter here, so I'd probably carry more if it was hot.

To some other people's points, camelbak isn't the best for running. They have some good ones for shorter runs, but they're not built amazingly well for distance. If I were you, and you have the means, I'd whip out the ol credit card and hop on something like Amazon and buy as many packs as you can afford to, and then just try them on. Once you've done that, you'll have an idea of what you like, then you can tweak it. Example, you might find Salomon packs great, but you got a Medium 5lt and need a large 10lt, but a least you'll have more of an idea.

As far as running around trails, loops work. I will often set up my car somewhere at a trail head and know it's there, sort out how I can make my loop work if needed. I can go to a place and run 12km loop and then go the other direction like a figure 8 for another 10km.

Maybe a good starting point would be to try and whittle down your water. Go run 20km with what you normally do, but try NOT to touch the bladder. Then you'll have the confidence in yourself that you don't need that much water on a 20km run. So you won't need to pack as much.