@scotroutes I think a small reservoir may be the way to go. I get through quite a bit of water on summer days, and don’t like carrying a hard case bottle on my back in case I fall (have had a couple of sore ones in the past when I’ve slid and landed on my back and had things dig in when wearing a waist pack). And agreed, I keep my mountain bike reservoir water only, and let it dry properly when I get home.
For longer summer runs in the Pentlands I just tend to take foil blanket, light jacket and (sometimes) light hat and gloves, plus phone, food, keys etc. Doesn’t take up too much space but gives me that reassurance should I slip and twist and ankle or something. Exact kit depends very much on forecast though.