I'd recommend getting some heavy duty hessian bags or similar and fill with sand. Sits nice and snug in the pack and won't move around. They're cheap, and you can weigh and bag different weights so its easy to add or subtract as you feel. I got two 20 kg's and a couple of 5's still laying around in my garage.
Now, judging by your log, you are both fit and strong so "light" is probably very different for us hahaha...
i started with 10kg but quickly increased. Most importantly, the heavier you go, the less "ROM" you'll have when running. It becomes more like a shuffle. In saying that, I used an old school army pack with steel frame. Good to load heavy but not so comfortable to run with.
And yeah, I definitely felt sore in the whole body, legs, back and shoulders.