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I've been using one of these for long runs or fast hiking for the last 18 months or so. Has been great for me.
UTOBEST Running Backpacks Lightweight Hydration Pack Functional Running Vest 5.5L https://amzn.eu/d/276bB0U
Fit can be very specific to the individual for this sort of thing but I'm six four and relatively large chested and it fits me well and my wife is 5'3 and her one adjusts to fit her well too.
What bags do people use for longer training runs, for me that’s like 35km. Want to carry a couple bottles and some nutrition really. Got a last minute spot on the london marathon and need to ram in what training I can coming from a cycling background