In reply to higherclimbingwales: I think you're going to find horses for courses. Walking doesn't generate the heat tht running does therefore you tend to wear more layers and require a different fit to a running bag.
Walking I'd recommend the OMM12 as previously mentioned, light good capacity reasonable fit.
Running though the OMM is tiring and moves around too much. It also has a looser fit so can cause chafing from riing about. The hydration sack that's on everyone's (running) back is the Salomon S-Lab Skin (5 or 12) superb piece of kit. The 12 holds all the mndatory kit for long ultra races in the mountins, waterproofs, mps, head torch, first aid, hat/gloves, food, water etc.