I used a MULE for years when I was a keen mountain biker and it was a great day-back for this. However when I was marathon training I picked up a Camelbak Octane as it was much better suited to running.
Admittedly its a good few years ago that I bought the MULE so it might have changed. I would have found it too loose for running as its quite narrow (i.e. slim between shoulders) and quite deep (protruding from your back) which is great for cycling as it keeps the weight centred over your spine, but not so good for the bouncing up and down associated with running.
I'm very happy with the build quality of Camelbak products and it will stand up to showers etc, but its not fully waterproof.