Hello @Dog_Jogger !

To add to @REI-JohnJ 's and @REI-PearlD 's great suggestions, I just discovered the REI Co-op Link 15L Hydration Pack . I haven't used it for running, but I am impressed with how it stays in place while I am mountain biking, and the hydration bladder feels very comfortable in the lumbar area. I appreciate the breathable back panel, and it has great compartments for my necessities. I encourage you to take a look!

Enjoy the trails!